Maybe someone more familiar with the CM/router equipment can answer here, but in general I'd have to say DID from the CO is always going to be your best bet for inbound fax routing. We've had lots of fiddling/timing issues with having various PBXes forward us the DID info via DTMF (the NEC is a perfect example of that)... you can never go wrong with straight DID.
Of course your CPI card isn't going to handle that... you'll need a CPD220 or, since you're running fax off the Unity server, you can also use any number of Brooktrout DID capable fax cards.
Jeff Lindborg
Unity Product Architect/Answer Monkey
Cisco Systems
lindborg@cisco.com
http://www.AnswerMonkey.net (new page for Unity support tools and scripts)