In my adventures I ran into a SIemens Gigaset Bluetooth Adapter AKA Xlink
https://www.amazon.com/Siemens-Gigaset-Bluetooth-Gateway-Black/dp/B002M17BVG
I did not buy it but I know where the seller is, if someone is interested.
There was no price on it, and the seller accepts reasonable offers
It is the model with PSTN (Line) port and 3 Bluetooth connections.