Modem MAC MTA address

To save your sanity, don't.

Modems (and fax) over VoIP are extremely sensitive to timing, and usually break over VoIP, it might even seem to work at first, but it's certain to break.

Adding virtual machines just makes the problem worse.

There are "short haul modems" around which can emulate a phone line.

Another option is just to attach the two modems back to back:

http://www.jagshouse.com/modem.html

I'm not sure if there's a client, but it sounds like you should be able to use reverse telnet to a Cisco AS5300. The AS5300 takes a PRI (T1 with ISDN framing, so 24 phone lines worth) and should support dial-out (I think), in a single rack space.