Not trying to disapoint you or make you give up but I tried this with D-Link recommanded BT dongle and I never got it to work in 6 months and posting on various threads. The hassel was way too much and I give up. Probably a GSM gateway would be ideal. However, if you find an easy solution I'd luv to know about it.
I have a PORTech MV-372 GSM to SIP gateway that uses Siemens Quad-band GSM chip and has two slots for SIM cards. It is easy to get working and works well with Elastix. It's somewhat expensive (somewhere around $600).