There's a device called cell2jack that converts your mobile phone to a pseudo landline. It talks to the cell phone by Bluetooth and has an rj11 port that you plug a landline phone into.
I wonder if one can do similar with software on a raspberry pi. That is, use Bluetooth and a mobile phone instead of that board. It's clunkier but everyone has old phones around, that have the right bands etc. the Pi software could even create a listener port that you can connect to with a SIP client.
Is there a reason you don't want to just use a SIP service by the way? It would certainly be cheaper than that board plus a mobile plan.