FSK (Frequency-Shift Keying) and DTMF (Dual Tone Multi-Frequency) are data transmission formats that carry caller information.
DTMF transmits the caller’s number. FSK is a more recent format for the transmission of caller’s number (or caller’s number and name depending on the network).
Caller ID service depends on these data transmission formats being supported by the phone. In addition consumers need to subscribe to these services with their service provider.
Additional information: There are two types of caller information detection. The first type, type I, refers to caller ID detection when the phone rings from standby mode. Type II is the caller ID detection when the phone receives a second call during talk mode. Type II service may require additional subscription with the service provider.