Some time ago there was the DeDECTed project which used a Com-on-Air card to decode audio from DECT 6.0 cordless phones.
I have a video on this process here: BaudRate (a show I never finished) Decoding DECT.
DECT phone call decoding can be done much easier than it used to be.
Today all you really need is an SDR and GNURadio to do this, however scanning support isn’t really there yet.
I’ve taken GR-DECT2 and modified it a bit to add support for the US dect bands and I plan to add scanning support in the future.