Fax transmission has been designed and optimized to fully exploit the physical characteristics of traditional PSTN analogical copper lines, and has been the bête noire of VoIP for a long time. Even the best, uncompressed codecs (G711) are not able to guarantee a high success rate of T30 (for example, fax) transmissions. That's because of the hyperstrict timing requirements that were guaranteed by a real-time analogical transmission, but are practically impossible for an asynchronous digital transmission. We'll see this in a later section of this book, but the SIP solution to this problem is a protocol enhancement called T38. T38 works around the timing problems, but its implementation must be compatible end to end, and the eventual gateway to PSTN connected fax machines must be of high quality.

So, if you need faxes (inbound and/or outbound), choose an ITSP with well–known, good T38 support for routing this kind of traffic, and perform many tests before committing...