OBiBT Bluetooth Adapter for Pairing the OBi202 with a Mobile Phone Now Available

[HHinFlorida]

I think that you can. The voip service providers (SPx), can be bridged with the BT service. Have a look here.
Use Your OBi202 with OBiBT as a Cellular (GSM or CDMA/PCS) Mobile Phone Service Gateway for a SIP IP Phone
http://blog.obihai.com/2012/09/use-your-obi202-with-obibt-as-cellular.html

I think what Synapse is asking for is more like the inverse product.

That is to use the Obi202 with bluetooth as some kind of wireless base station for cordless housephones.

Now would that be new hardware, or is there a hope that it could be implemented as a firmware change alone?

[Ostracus]

Cordless phones use DECT, not BT.

[giqcass]

What he asks would require special software on the phone as well as the Obi.  It could be done but I don't think it's a worthwhile project.  A re-tasked femtocell would make more sense.  The easy solution would be installing Obi or sip software on the phone.