Might be a little late but...

We are doing this now and was doing it on an AXXESS system as well.

Inter-tel, er Mitel, is horrible about sending down DNIS and CID to analog ports. So what I did was create a phantom ext for each of the DID numbers. So lets say the DID is 555.1234 i created a phantom ext of 171234 and had that forward to a hunt group of my analog ports that our fax server is on. The fax server can see the "CID" of the call being 171234 so i can route my faxes to users based upon that. This works well so far for 100+ fax users but not my ideal solution. For us the next step is using a PRI for our fax server by itself.