You would think there would be a box that would connect a POTS phone to the air interface of an existing DECT base station, but I haven't seen one.
There is this though.
https://www.amazon.co.uk/RTX-WIRELES...hone+extension
It is DECT, but I believe it only works with its dedicated master unit, and cannot be paired to a normal base station.
If your phone uses loop disconnect dialling, I doubt very much it will work with the above unit, however there are several options for converting "rotary" phones to DTMF
eg RotaTone or DialGizmo