SIM Application Toolkit (STK)
Updated
The SIM Application Toolkit lets applications stored on the SIM act on the device: send an SMS, refresh files, select a network or display a menu. It is specified by ETSI TS 102 223 as the Card Application Toolkit and, for 3GPP networks, by 3GPP TS 31.111.
In practice
Many network switching features of IoT SIMs are toolkit applications. They work only if the device’s modem supports the toolkit commands they use, which is worth testing before a large rollout.