FSK is Frequency-Shift Keying and DTMF is Dual Tone Multi-Frequency, they are data transmission formats that carry caller information and caller ID detect systems. 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 end users need to subscribe to these services with their local service provider.