This attribute specifies whether ingress fax idle suppression is
desired during G.711 and G.726 fax traffic. It applies only if
modemFaxEncoding is g711G726 or faxRelayG711G726.
The negotiated ingress G.711/G.726 fax idle suppression is
available in negotiatedIgFisG711G726.
If the silenceSuppressor or vtds feature is present,
faxIdleSuppressionG711G726 defaults to on. Otherwise,
faxIdleSuppressionG711G726 defaults to off.