This attribute specifies whether this VoiceService differentiates
modemFax traffic and speech. If modemFaxSpeechDiscrim is on
and a false 2100 Hz tone is detected, this VoiceService reverts to
speech mode when speech is detected.
modemFaxSpeechDiscrim is not used if this VoiceService is running
on a DS1V, E1V, 1pDS1V, 1pE1V and J2MV card.