PhonePower, the guys I'm with, have this as an option. It used to be offered as standard as on my previous provider, Broadvoxdirect, prior to them selling off their residential business to PhonePower.

When I fist joined Broadvoxdirect, it used to work transparently. Basically the call would go to voicemail and if it heard a carrier it would treat it as a fax. Later they changed to prompting callers whether they wanted to send a fax or leave a voice message. I'm not sure how PhonePower's system handles it. The fax would then be delivered via email as a TIFF attachment - I also received all voice mails by email as well as wav attachments (still do right now too).

There's a button to enable it in my current control panel, but I'm not sure if it's a pay option. I'm currently waiting for them to fix a couple of caller ID issues, so I don't want to test any other options out right now.