AN97 Description
SIGNAL GENERATION CIRCUITS Application Note 97: Jul 09, 1998 Telephone Tone Generator Requires No Trimming Many products that connect to phone lines (modems, for instance) incorporate a "callprogress monitoring" function known as CPM. CPM circuits "listen" to the lines as a human would, and respond according to what they "hear." You shouldn't dial a number unless you first hear a dial tone, for example.