Pay as you go (phone) plans

When traveling I prefer to use local sim cards on a 'pay as you go' type plan. These plans are quite popular in Thailand and readily available. Other countries like Taiwan place some restrictions but in combination with Skype for internal calls, it's a far more economical alternative to roaming.

The following provided by wikipedia is a short introduction to the concept:

Pay As You Go, often shortened to PAYG, is used as a general term for the concept of a prepay mobile phone. In many countries this is referrred to as "prepaid" mobile service.
The concept was initially developed by Eircell in the Republic of Ireland in the 1990s, as a method of letting different types of people (those under the age of 18, those without bank accounts and those without proof of identity) obtain a mobile phone. Originally limited to one TACS handset, costing £99 upfront, the system was an amazing success, despite the high price of calls and a 7p service charge on every operation. The system was branded as Ready To Go, a name still used by Vodafone, who now own Eircell.
A user would buy a phone, usually pre-loaded with some amount of credit, and would purchase extra credit when required. A call cannot be made unless the user has the amount required for that call's minimum charge. Some networks charge more for the credit than you get in call value, often due to service charges and VAT on phone calls; however the opposite is often true, with users paying, for example, €20 for €22 call credit.
The concept has since been copied in many other countries, with virtually every network in every European country supporting it. On many networks, such as Ireland's Meteor, Pay As You Go is the main mode of operation, with account phones being very much second-class. Conversely, in the United States, account phones offer the best features with pay as you go services being far more restricted in functionality.
Often, Pay As You Go customers pay more for their calls and SMS messages, and are limited in what they can do with their phone - calls to international or premium rate numbers may be blocked, and they may not be able to roam. These limitations are often due to the complexity of managing the credit system for high price calls, or when the user was not on their home network.
In the United Kingdom, operators have started placing restrictions on PAYG users (for instance, having a minimum topup amount of more than £10) and raising the price of PAYG phones in an effort to attract users to contract plans (which tend to earn more money than prepay overall).

Pay as you go (phone) - Wikipedia, the free encyclopedia