Hi, and welcome to OBi-land!
You can either use your already-configured SP1 Google Voice number for faxing, or you can get another Google Voice number (you'd need a separate Google Voice account, and a separate forwarding phone number to request another GV number), and configure it as SP2 on your OBi, or you can get a free inbound number from Callcentric, and configure that on one of the OBi's SPx slots.
If you decide to use your SP1 GV number, you can plug the OBi's Line 1 into your fax machine (use a two-wire, RJ-11 cable, not a four-wire, RJ-14 cable). You'd then plug your telephone cord into the fax machine's phone jack, and set up distinctive ringing on the fax and on the OBi.
RE: your DECT phone, your comment is unclear. What, exactly, says "Connecting..."? The display on the DECT phone's handset? If so, you either have plugged the phone into the wrong jack on the back of the OBi, or your phone cord is bad, or your handset has lost its registration to its base -- not a OBi problem. Try some other phone, preferably a hard-wired phone.