If you are using Fax Over IP:-
The IP Office has a proprietary method for carrying Fax traffic on a VoIP call, it is supported between IP Office systems and supports Fax speeds upto 14.4 kbps, it does not confirm to the T.38 standard. The bandwidth requirements for the call will initially be as per the specified or negotiated compression method then the bandwidth requirement will change to accommodate the Fax data. The Fax bandwidth will vary depending on the speed that the Fax devices are communicating at and the type of link, at 14.4 kbps the bandwidth requirement will be approximately 27 kbps on the LAN or 19 kbps on a Point to Point WAN link with Header Compression.