Thai mobile networks will probably not offer the type of reliability that you are used to in your home country. If you wish to make and receive calls at any time in Thailand, you will probably need at least 3 SIM cards from different providers.
While there is a vast range of companies that sell mobile phones, only three companies provide mobile services:
The service, signal quality, coverage, and support vary even within the same company. You can never be 100% sure that your mobile line is going to work, so it is better to have an additional mobile line. If your voice calls can not get through to the other line, in some cases an SMS will.
Charges for a domestic call to or from a mobile phone vary, according to which calling plan you have. The usual rate is 3 Baht per minute, but can range from 1 to 6 Baht.
When you buy a mobile phone, you buy two things: the phone and a phone number. The phone number is embedded on the SIM card which you slide into the phone.
Mobile phones can be bought at any shopping centre or small shops that sell mobile phone units. SIM cards are sold at service provider shops throughout the country, but also at convenience stores such as 7-11. However, you will have to register your SIM card at a branch of your service provider, so it is easier to buy a SIM card immediately at a service provider branch.
There are two different types of SIM card available:
If you loose your mobile phone in Thailand, a new SIM card will be supplied either for free or for a small fee.
If you already have a mobile phone with an international roaming service, it will usually work in Thailand. Be aware that if you use an international roaming service, your phone bills are going to be astronomically high. It is best to get a local mobile line if you are staying for an extended period of time in Thailand.