Fax over IP is a struggle. Still. Not incoming, but outgoing. Even with T.38 enabled sometimes. Clients often ask for solutions to the Faxing over IP. Here is a webinar from Biscom about How to Reduce Your Fax Costs with Virtualization, FoIP and Cloud Faxing on Thursday, September 22, 2011 Time: 12:00 PM EDT/ 9:00 AM PST. Register here.
There are plenty of efax services from J2, conferencing companies like PGi, and even my pal at FaxBetter.com.
Here's how one CLEC handles the FoIP issue: Sonic.net offers free FaxLine service.