Signal decoders used in communication systems, such as telephony and two-way radio systems, detect communication protocol states by recognizing a variety of electrical, optical, or acoustic conditions.
Proper detection of signaling is a compromise between acceptable error rates and cost of implementation.
Two-out-of-five code and similar methods provide an additional check in some applications.
In the case of Quik Call I or a string of DTMF digits, the falsing would have to occur in the exact order required to actuate the decoder.
Examples of systems of two-tone sequential paging are Motorola Quik Call II, General Electric Mobile Radio Type 99, and later-model Plectron receivers.