This read-write setting chooses whether this voice channel handles fax calls, modem calls, both, or neither, via the none/fax/both options. Enabling fax or modem relay tells the unit to switch the channel's handling mode when it detects fax or modem tones instead of treating the call as ordinary compressed voice, which is important because low-bit-rate voice codecs distort fax and modem signals. For example, a port that regularly carries office fax machine traffic alongside phone calls would be set to both(3) so either call type is handled correctly.