Faxing problems. SIP trunks, shoretel, and a Multitech faxfinder

We currently cannot send or receive faxes. We have a Multitech FaxFinder-240IP-2 fax server appliance paired with a ShoreTel phone system (v. 14.2). Here is what we know: • When an incoming fax arrives, our provider sends it to our SIParator device which routes it to our ShoreTel PBX. • Once it’s handed to ShoreTel, its routed over a vtrunk via “off system extension” to the FaxFinder appliance. • It reaches the FaxFinder appliance correctly, but gets hung up at “negotiating” and the call/fax fails to complete. The error descriptions in the logs are generic and provide little to no detail on a possible solution. I’m happy to provide Wireshark packet captures that we’ve gathered while troubleshooting. Any help would be greatly appreciated.