This read-only INTEGER determines how the echo canceler behaves in the presence of a 2100 Hz modem answer tone: ignore it, disable for all tones per G.164, or disable only for phase-reversing tones per G.165 (the reserve value is not a valid setting and returns an error). This matters because leaving the canceler active during a modem/fax negotiation can corrupt the data signal, so an admin needs the right mode selected for lines carrying fax or modem traffic. For example, a line that regularly carries fax calls would typically use the g-165 mode so the canceler only backs off during the specific tones that signal a modem handshake.