Here what I used in my trip to Portugal in Jun2013, in case others find this helpful:

Disclaimer:

• In the US, buying a cellphone “out-of-contract” is not the same as buying a cellphone “unlocked”. All of the following only works for an unlocked phone. Make sure your phone is unlocked before you get on the plane.
• Different cellphone companies have different policies on this. AT&T declared that, despite my being a multi-year customer, with no contract, they would not unlock my phone per policy. T-Mobile said upfront that they would need ~40days from date-of-purchase of “out-of-contact” phone before I could ask to have it unlocked. On the 40th day, when I asked T-Mobile to unlock my phone, they sent me the phone unlock codes within 48hours.
• Make sure your phone supports GSM. Sounds obvious, but still needs to be said, as most countries use GSM.

• Buy a “LycaMobile” pay-as-you-go SIM card. I bought mine at the train station in Lisbon, but they are also for sale on most small street corner stores. While there are several mobile companies selling pay-as-you-go, I went with Lycamobile because they had the best price for all-you-can-use data at 4G speeds, great high speed coverage everywhere I went, and no hassle about using your cellphone as a hotspot. Oh, and comparable prices for voice calls and text messaging.
• Disassemble your phone to swap out sim card, insert new LycaMobile sim card and power up the phone.
• On the phone, enter “*#123#″ and press dial (typically, the green handset button). This connects you to an automated service that tells you your balance.
• To find out what your lycamobile phone number is, dial “*#122#”
• Assuming that all works, you should now attempt to call any local number. By habit, I call the mobile phone of the person at the store selling me the SIM card.
• Cultural tip: I never setup voicemail – as discovered in my other recent trips, most people dont both leaving voice messages on cellphones anymore – if they cant reach you when they phone, they hangup and send you a text message instead.
• Now that you can make/receive calls, to make my Android 2.2 phone transmit/receive data, I had to add the following APN settings:
  * on home screen, go into “settings”
  * go into “wireless & network settings”
  * go into “mobile networks”
  * go into “access point names”
  * if there is not already a “data.lycamobile.pt” APN, then create one as follows:
  ** Name = data.lycamobile.pt
  ** APN == data.lycamobile.pt
  ** Proxy == Not set
  ** Port == Not set
  ** Username == impt
  ** Password == impt
  ** Server == Not set
  ** MMSC == Not set
  ** MMS proxy == Not set
  ** MMS port == Not set
  ** MCC == 268
  ** MNC == 04
  ** Authentication Type == Not set
  ** APN Type == Not set
  …hit save, and go back to “Access Point Names”.

• verify that this new “data.lycamobile.pt” APK is present, and is selected.
• verify that “Use only 2G networks” is not selected.
• Reboot the phone to see if that helps.
• At this point you should be able to make/receive calls, send/receive text messages, surf the web, use your cellphone as GPS, and use your cellphone as a wifi hotspot.
• To check your account balance dial “*122#”.
• When you need additional credits, buy a one-time use scratch-refill “top up” card at almost any corner store, and follow the instructions on the back. You’ll receive a text message with the new balance when the credits are added to your account.
  

Here what I used in my trip to Malaysia in Nov2012, in case others find this helpful:

Disclaimer:

• In the US, buying a cellphone “out-of-contract” is not the same as buying a cellphone “unlocked”. All of the following only works for an unlocked phone. Make sure your phone is unlocked before you get on the plane.
• Different cellphone companies have different policies on this. AT&T declared that, despite my being a multi-year customer, with no contract, they would not unlock my phone per policy. T-Mobile said upfront that they would need ~40days from date-of-purchase of “out-of-contact” phone before I could ask to have it unlocked. On the 40th day, when I asked T-Mobile to unlock my phone, they sent me the phone unlock codes within 48hours.
• Make sure your phone supports GSM. Sounds obvious, but still needs to be said, as most countries use GSM.

• NOTE: You need to show your passport, or national ID card, when buying a pay-as-you-go SIM, or a phone, in Malaysia.
• Buy a “HotLink” pay-as-you-go SIM card. I bought mine at the airport in Kuala Lumpur, but they are also for sale on most small street corner stores. While there are several mobile companies selling pay-as-you-go, I went with HotLink because they had the best price for data at 4G-LTE speeds, great high speed coverage everywhere I went, and no hassle about using your cellphone as a hotspot. Oh, and comparable prices for voice calls and text messaging.
• Disassemble your phone to swap out sim card, insert new HotLink sim card and power up the phone.
• On the phone, enter “*122#″ and press dial (typically, the green handset button). This will connect you to an automated voice service which will tell you your balance.
• Assuming that works, you should now attempt to call any local number. By habit, I now call the mobile phone of the person at the store selling me the SIM card.
• Cultural tip: I never setup voicemail – as discovered in my other recent trips to SEasia, most people dont both leaving voice messages – if they cant reach you when they phone, they hangup and send you a text message instead.
• At this point you should be able to make/receive calls.
• To make my Android 2.2 phone transmit/receive data, I had to add the following APN settings:
  * on home screen, go into “settings”
  * go into “wireless & network settings”
  * go into “mobile networks”
  * go into “access point names”
  * if there is not already a “maxis” APN, then create one as follows:
  ** Name = maxis
  ** APN == bbnet
  ** Proxy == 202.75.133.49
  ** Port == 80
  ** Username == Not set
  ** Password == Not set
  ** Server == Not set
  ** MMSC == Not set
  ** MMS proxy == Not set
  ** MMS port == Not set
  ** MCC == 502
  ** MNC == 12
  ** Authentication Type == Not set
  ** APN Type == Not set
  …hit save, and go back to “Access Point Names”.

• verify that this new “maxis” APK is present, and is selected.
• verify that “Use only 2G networks” is not selected.
• Reboot the phone to see if that helps.
• Now that the phone is configured correctly, I selected the 500mb-per-day data plan, as follows:
  enter “*100*9*1#” and press dialer
  read menu, enter “2” and click “ok”
  read menu, enter “1” and click “ok”
  …thats it.

• At this point you should be able to make/receive calls, send/receive text messages, surf the web, and use your cellphone as a wifi hotspot.
• To check your account balance call *122#.
• When you need additional credits, buy a one-time use scratch-refill “top up” card at almost any corner store, and follow the instructions on the back. Alternatively, their website says you can topup using PayPal as well as credit cards, but I never personally tried that. Either way, you’ll receive a text message with the new balance when the credits are added to your account.
  

Here what I used in my trip back to Ireland in Oct2011, in case others find this helpful:

Disclaimer:

• In the US, buying a cellphone “out-of-contract” is not the same as buying a cellphone “unlocked”. All of the following only works for an unlocked phone. Make sure your phone is unlocked before you get on the plane.
• Different cellphone companies have different policies on this. AT&T declared that, despite my being a multi-year customer, with no contract, they would not unlock my phone per policy. T-Mobile said upfront that they would need ~40days from date-of-purchase of “out-of-contact” phone before I could ask to have it unlocked. On the 40th day, when I asked T-Mobile to unlock my phone, they sent me the phone unlock codes within 48hours.
• Make sure your phone supports GSM. Sounds obvious, but still needs to be said, as most countries use GSM.

• Buy a “Three.ie” pay-as-you-go SIM card. I bought mine at a stall in a shopping center in Dublin, but they were also for sale in any of the countless newsagent stores across the country. While there are several mobile companies selling pay-as-you-go, I decided to go with Three.ie because they have unlimited(yes, really!) data at 3G speeds, no hassle about using your cellphone as a hotspot and unlimited text messaging. Oh, and comparable prices for voice calls.
• Disassemble your phone to swap out sim card, insert new Three.ie sim card and power up the phone.
• On the phone, enter “1744” and press dial (typically, the green handset button). This will connect you to an automated voice service which will tell you your balance.
• Assuming that works, you should now attempt to call any local number. A good example is the mobile phone of the person at the three.ie stand who sold you the card. Specifically, you should do this because the first call made on the new three.ie account will not be connected until you are verbally prompted through the remaining setup steps. This includes setting up an access PIN (for later topup/account activity calls) and also setting up voicemail, all of which I did.

  (Cultural note: While some people would leave voicemail, most people would instead send text messages. However, I found it interesting that even people who didnt leave a voicemail would still want to hear my voice on my voicemail greeting, to confirm they had reached the correct number, before they would hangup and send me a text message.)

• At this point you should be able to make/receive calls.
• To make my Android 2.2 phone transmit/receive data, I had to add the following APN settings:
  * on home screen, go into “settings”
  * go into “wireless & network settings”
  * go into “mobile networks”
  * go into “access point names”
  * if there is not already a “smart” APN, then create one as follows:
  ** Name = 3 services
  ** APN == 3ireland.ie
  ** Proxy == Not set
  ** Port == Not set
  ** Username == Not set
  ** Password == Not set
  ** Server == Not set
  ** MMSC == Not set
  ** MMS proxy == Not set
  ** MMS port == Not set
  ** MCC == 272
  ** MNC == 05
  ** Authentication Type == Not set
  ** APN Type == Not set
  …hit save, and go back to “Access Point Names”.

• verify that this new “3 services” APK is present, and is selected.
• Reboot the phone to see if that helps.
• At this point you should be able to make/receive calls, send/receive text messages, surf the web, and use your cellphone as a wifi hotspot.
• To check your account balance call 1744.
• When you need additional credits, buy a one-time use scratch-refill “top up” card at almost any newsagent, and follow the instructions on the back. Alternatively, you can buy additional credits by calling 1744 from your phone, enter your PIN and recharge using a credit card. Either way, you’ll receive a text message with the new balance when the credits are added to your account.
  

There are now 9!!?! different mobile phone companies in Cambodia and a great summary of them all is on khmerbird.com. They all offer different pay-as-you-go SIM cards, and of course, there are lots of different Android phones, but here what I used in my recent trip back to Cambodia, in case others find this helpful:

Disclaimer:

• In the US, buying a cellphone “out-of-contract” is not the same as buying a cellphone “unlocked”. All of the following only works for an unlocked phone. Make sure your phone is unlocked before you get on the plane.
• Different cellphone companies have different policies on this. AT&T declared that, despite my being a multi-year customer, with no contract, they would not unlock my phone per policy. T-Mobile said upfront that they would need ~40days from date-of-purchase of “out-of-contact” phone before I could ask to have it unlocked. On the 40th day, when I asked T-Mobile to unlock my phone, they sent me the phone unlock codes within 48hours.
• Make sure your phone supports GSM. Sounds obvious, but still needs to be said, as most countries use GSM.

• Buy a “Smart Mobile” pay-as-you-go SIM card, with the “Fresh Internet+” plan. I bought mine at the airport in Phnom Penh, but they were also for sale in any of the countless green “Smart Mobile” stores across the country. While each of the 9 cell phone companies in Cambodia have different voice prices, this company seemed to have the best data plan, which is what I cared about most. Oh, and the staff at their booth were great fun and their ad for “Fresh Internet+” data-plan looked fun!
  

• Disassemble your phone to swap out sim card, insert new SmartMobile sim card and power up the phone.
• On the phone, enter “*888#” and press dial (typically, the green handset button). This should send you a text message within a few seconds containing:
  * your mobile number
  * remaining balance on your SIM card
  * expiry date of the credits on your SIM card
  * whether or not you have a data plan set up (the default is “not”).

• At this point you should be able to make/receive calls, and check your account balance.
• Dont bother setting up the voicemail. Like in Hong Kong, no-one seems to use voicemail; if you dont answer the phone, callers simply try again later or send you a txt msg.
• To enable your phone for pre-paid data, you need to setup “Fresh Internet+” plan, as follows:
  * To verify data plan status, enter “*093*0#” and press dial. It should tell you that you have no data plan.
  * On the phone, enter “*093*3#” and press dial (typically, the green handset button). This cost me USD$5, and gave me 2GB data limit for 30days. (Click here for list of codes for different prices and different data limits.). All the SmartMobile plans are 30day duration, the only difference is how much money you want to prepay, and what data limit that gives you.
  * You should receive an SMS confirming the amount paid, and the data plan.
  * To verify data plan status, enter “*093*0#” and press dial. It should tell you that you have a data plan, days to expiry, and how much of your data plan allowance remains.

• The following APN settings were enabled automatically on my Android 2.2 phone, so I did not set these manually. However, I’m listing them below in case it helps others debug problems transmitting/receiving on their data plan:
  * on home screen, go into “settings”
  * go into “wireless & network settings”
  * go into “mobile networks”
  * go into “access point names”
  * if there is not already a “smart” APN, then create one as follows:
  ** Name = smart
  ** APN == smart
  ** Proxy == Not set
  ** Port == Not set
  ** Username == Not set
  ** Password == Not set
  ** Server == Not set
  ** MMSC == Not set
  ** MMS proxy == Not set
  ** MMS port == Not set
  ** MCC == Not set
  ** MNC == Not set
  ** Authentication Type == None
  ** APN Type == default
  …hit save, and go back to “access point names”.

• verify that this new “smart” APK is present, and is selected.
• Reboot the phone to see if that helps.
• When you need additional credits, buy a one-time use scratch-refill card at almost any roadside stand (they are literally everywhere, and all have the distinctive green “Smart Mobile” logo or beach umbrella!).