My two cents. I'm heavily involved with incoming/outgoing faxes. Almost all offices need this functionality & I have requested ALL clients, for sake of simplicity to stick with a regular PSTN to fax machine. That's my two cents.
To describe in details... my experience for: PSTN <==> VOIP Server 1 <==> VOIP Server 2 <==> ATA <==> Fax Machine *almost* works. I don't like the word impossible :). For a fax that takes 30 seconds to go, ends up taking about 2-3 minutes per page... and in some cases 4 minutes. So the time and aggravation is not worth in my humble opinion. I also know of a business first hand, running Asterisk and receiving faxes, converting them to TIFF files and sending via e-mail. Their set up is PSTN <==> Asterisk Server <==> E-mail recipient. That works well, and the company has over 100 lines, with 4 T1's to one box. I have also personally experimented with friends overseas and the setting was like this: Their Fax <==> Their ATA <===> Asterisk Server <==> My ATA <==> My Fax and this works well. Just my 2 cents to shed some light. Cheers! Reza.
