T.38 is a protocol that describes how to send a fax over a computer data network. T.38 is needed because fax data cannot be sent over a computer data network in the same way as voice messages. The T.38 protocol allows fax to travel over a VoIP network. T.38 is also used to terminate a FAX transmission at the boundary of the VoIP network so that packet loss in the IP network does not affect FAX calls.
Messaging can receive and forward faxes to an email inbox you specify through the inbound (native) fax. Messaging can also detect and transfer faxes to a third party fax server. This FAX transmission depends on CNG, the calling tone, detection and T.38 retransmission. For the "detect and transfer" behavior of Messaging, after CNG is detected, the Messaging system initiates a transfer of the call to the third party fax server. For native fax, after CNG is detected, the Messaging system initiates T.38 session with the telephony server. The telephony server must be configured to support T.38 protocol.