Can voice calls from/to the PRI be routed to and from the FXS ports without a SIP trunk?
Model: Total Access 908
Absolutely. You'll want the dial plan set correctly and a grouped-trunk matching what the PRI expects assigned to its trunk.
Although not a roadblock, the PRI to the PBX operates as a network. Can it operate as a user?
As a rule, PBXs are role user and the device feeding them is role network. You could flip them if your PBX is capable of being set to network role. It won't make a lot of difference as long as one end is role user and the other is role network. Both user or both network will not work and you'll see a lot of debug output events if you try it (so don't do that then).
Another gotcha are that the switch type should match on both sides (best to use National NI-2).
Also ensure that exactly one end of the T1 is supplying clock and the other side clocks from the line. Like network/user it doesn't really matter which is which unless either device is connected to other T1s than the one between them. Then it can get complicated.