This attribute specifies the maximum possible bit rate (minimum
possible compression rate) during a modem call on this channel. It
also specifies the maximum possible bit rate (minimum possible
compression rate) during a fax call on this channel, but only if
faxEncoding is g711G726. maxModemFaxG711G726Rate always
applies to modem traffic, and never applies to voice traffic. If
faxEncoding is g711G726, maxModemFaxG711G726Rate also
applies to fax traffic. If faxEncoding is faxRelay,
maxModemFaxG711G726Rate does not apply to fax traffic.
Rates can change during a call due to congestion in the network.