Another solution might be to have a BT access point or BT on a SIP/Asterisk server. You drop the phone near the BT and route calls through a SIP service. In other words the cell ties to a SIP server and your NIT (or any other SIP client on PC etc) can use the line.

I'm thinking of doing this mainly for inbound calls that happen to come in on a cell #. (I don't own a cell but the workmates do). Outbound on SIP via other trunk lines are usually much cheaper. The cell becomes a trunk line for the SIP server.

This isn't exactly what you were asking but is what I'm planning. The idea is that people drop their phones near my server when they come to work and then take them home later. The deskphone and NIT can take regular calls as well as their cell calls.