Hmmm. I don’t know if DECT base on Sangoma will be enough for you. If not, there’s the SNOM 700.
I know there’s another brand with the same spec a kind of clone of SNOM 700. That works on FreePBX.
The clone of SNOM 700 is RTX 8660. Depending the country you are, you could buy it as well.
In France, there’s no way to get it right now, this device is no longer distributed.