Need faxing solution and ideas

I am in search for solution that allows our hosted customers to use faxing from our datacenter. They need to have ability to send and receive while forwarding their line to us. We mimic the phone number when faxes are sent out, they forward line to us - The world thinks they have their own fax machine, but all operation happens at our datacenter. This needs to be developed based off of hundreds of accounts, all having special dedicated line, folder, access, etc.

Here is the idea/need - details.

The customer has a fax line that is already been shared with the world. They use our software, we host, and deliver the our applications over internet. The software can send and receive faxes electronically, bringing the faxes into the software, but only if installed client/server.

We have SDK of taxman, but still are running into the issue of customers that want to use faxing, and running into problems due to our developers not that great with faxing solutions.

I have PRI at datacenter, MiTel unified phone system available - DID numbers, etc.

The goal here is to forward the customers faxes to the datacenter, where the application servers can monitor the faxes coming in, separate and tag them to the customer's account, and deliver to the application so the customer can see they have a new message/fax.

This also needs to work in opposite. The customer needs to be have ability to fax off the hosted system, but allowing the number to be tagged with their local number. When someone receives the fax, it says it comes from the customer's fax machine, even though it didn't.

Kind of difficult to go into details, but this is dedicated for healthcare business/software - Have potentially 1000-5000 faxes per day - Also not sure how to handle the billing - Long distance, fee per line, per page, etc.

Any ideas here would be great - Not sure if this is all possible. Thanks!