Hi!
The only thing I can see different from SIP trunks which are correctly shown as registered on my FreePBX box is the fact that they ask you to provider User Context and User details.
Non SIP trunks never show up in those stats for me but my SIP trunks always show up there (but none of them have a User Context and User Details, they all have only a register string for Incoming...).
Good luck and have a nice day!
Nick