A lot of people ask about faxing. The best way to go is with PSTN lines to guarantee inbound and outbound faxing. With Voip, there is really no discussion until asterisk 1.6 is released. There is no Voip provider that I know of who will guarantee outbound faxing right now. However, asterisk 1.6 has t.38 support which will even work with latency. Until asterisk 1.6 is released, in my opinion, there will be no fax support. But even when fax support does come out, it will only be available to asterisk 1.6 users, in my opinion- since asterisk 1.6 is supposed to have t.38 support.
In the meantime, you can go with Vitelity's e-fax service. They are cheap and charge per minute. You can send outbound faxes with their service, as well as receive incoming faxes to your email. You can do the same with Teliax, but you cannot send faxes. An advantage with Vitelity's e-fax, however, is that you can resell it to other customers and they will have their own e-fax login page. They will have no idea that you are reselling Vitelity' service.
I have done testing with trixbox and faxing with Voip providers and I have not found ANY reliable provider. They all produced the same results, where not even 50% of faxes would send/receive successfully.
For those users, like me, who do not want fax via email, and just want to use a regular fax machine, just get a regular landline like AT&T just for faxing. I have a pay per minute plan with AT&T. I pay about $30 per month, incoming faxes are free, and outgoing are 6 to 7 cents per minute. I hardly send out faxes. Trust me- this sounds expensive, but it's worth it.
I have heard from a few people that Gafachi is a Voip provider that offers reliable faxing. However, I have not tested them out. If you would like to test them out, make sure you are using asterisk 1.4 or later.