The new standard, t.38, was first deployed in 1998. The first phase of the deployment targeted intra-company use. Phase 2 is now underway for communicating using SIP trunking carriers for inter-company delivery.
The problem with fax-over-IP is not with the t.38 standard, but when the FAX transmission is made through multiple carrier networks, there can be delays introduced as each handoff is made. Voice is easy, because it doesn’t have the critical timing components that are a part of and essential to faxes. Some G.711 passthrough works on some highly managed networks. Through extensive testing, they discovered that one of the major problems is getting calls set up and transitioned from G.711 to T.38 because the media and signaling pathways are different.
The SIP Forum and the I3 Forum are cooperating to find a solution to the FAX over IP issue.