SIM Toolkit - A 1995 standard for VAS and transactions via mobile phone. The STK is programmed into the user's SIM card enabling the SIM to drive a handset's user interface. The Toolkit application can be updated OTA by the network, giving the handset some customisable functionality. The first commercial launch of STK services was from the UK's BT Cellnet in 1997. Since then it has become ubiquitous across GSM networks and is a key driver of the explosion of SMS usage with a primary use being to request or send information to the network via the SMS channel.

The SIM Application Toolkit is a mechanism that lets you use your SIM card to customise your phone's operation. By running an application resident in the SIM card's memory, the toolkit allows you to define certain phone functions, such as what appears on the display and your menu structure.