The country code for Thailand is 66.
Thai land-line telephone numbers are all nine digits in length while mobile numbers have ten digits, with the first digit in both cases always being zero. When dialing Thai numbers from outside of Thailand, you always drop the leading zero. Within Thailand, you must dial all nine numbers, even within the same area code.
The first two to three digits indicate the area code where land lines are located. For example, Bangkok numbers begin with 02, northern Thailand numbers begin with 05, with Chiang Mai numbers beginning with 053. Mobile phone numbers mostly begin with 081, although 086 is also quite common.
Pre-paid mobile phone cards are available at most any phone shop, as well as the airport arrivals area. Refills may be purchased at any 7-11 shop, as well as most other convenience stores.