Hey everyone, we have lots of customers using SPA devices. 112 all the way through the 8000 model. As far as faxing goes....
The official word is that we don't support fax over SIP. However, all of our trunks will support both T38 and G711ulaw for fax services. The issue with fax over SIP is that if your device or the other end does a subsequent INVITE message while you are faxing, it will kill the fax transmission. That's why we offer the alternative solutions (Fax2Email and Fax ATA).
I will be posting an SPA guide to our device guides very soon!