We want all students to have a cell phone for safety and because it is useful and convenient.
We recommend using AIS for cell phone service; they have good service and three special SIM packages for travelers.  For groups of students staying longer than 11 days, we like to recommend that you each get one Traveller Group SIM ( Traveller Sim 199 Baht) for the duration of your stay in Thailand.  This will allow you to keep the same phone number during your whole time in Thailand.  That is because it is able to be topped up (The Traveller Group Sharing SIM cannot be topped up). The Traveller Group SIM (199 baht) you can call free between Traveller Group SIM users for 7 days; after that the rate will be 1 baht per minute for all networks.  The SIM has 3G internet max speed for 500 MB, then the speed will reduce to 64 kbps. Also you can top up and add an internet package of 1.5 GB with max speed then the speed will reduce to 64 kbps for 10 days for 270 baht (including VAT). You can add this package by press *777*702#. You can purchase this sim card at all 7-11 branches.
Traveller Group Sharing SIM 459 baht promotion will end after 7 days and calling to all numbers will be 1.07 Baht per minute, internet 2.14 Baht per MB, and after 11 days your number account will be suspend. This card cannot add an extra package.  These SIMs also allow you to call home relatively inexpensively. They have regular and mini SIM cards.
Traveller SIM card and Traveller Group SIM card will have 2 sizes of SIM card in the package, Normal size and Micro size. For the Traveler Group Sharing SIM, there will be 3 sizes in the package: Normal size, Micro size and Nano size. But you can contact the branch with the SIM card to change the SIM size without cost.

Although Skype is available some people like to use their cell phone for calls home.  A CAT Phone Net card is useful.  Yupin has some for 300 baht. In the United States, mobile phone service providers usually lock phones into a promotion.  By law, they are required to unlock the phones if you will be traveling outside the United States. Almost all cell phones can be unlocked in Thailand.  Some are expensive to unlock ($100) and some take a couple days.    A cheap local phone costs 600 baht.  A SIM card can be 60 baht

Cell Phone Frequencies in Thailand