Over on the VoiceOps listserv, it seems that network admins of ITSP's prefer the analog POTS line for fax due to the headache and customer experience level. (Go with what we know works and less customer dissatisfaction).
Premiere offers efax services for service providers (and everyone else). Premiere is one of the 20+ carriers I rep. Another carrier, AstroTel, has a SmartFax solution that I currently use for inbound faxes. He also has an appliance for outbound faxes. My outbound solution is to scan and email.
WebWorkerDaily has an article about the 5 Alternatives to Keeping a Fax Machine, including some free options.
Do you have a page on your website that chronicles fax solutions for your customers?