http://www.rfc-editor.org/rfc/rfc4161.txt
RFC 4161: Guidelines for Optional Services for Internet Fax Gateways, K. Mimura, K. Yokoyama, T. Satoh, K. Watanabe, C. Kanaide2005-08-01T23:00:00-00:00This document supplements the recommendation for minimal features of an Internet Fax Gateway. In particular, it covers techniques for dropping duplicated fax messages, automatic fax re-transmission, error, return notice, and log handling, and possible authorization methods by DTMF (Dual Tone Multi-Frequency) for onramp gateways. This memo provides information for the Internet community.