Cordless phones have mostly disappeared but the Panasonic Link2Cell models - where the base station acts as a Bluetooth headset for up to two mobile phones - are still around and are absolutely marvelous for older adults. They can plug their mobile phone in to charge and walk around with one of the handsets (a base station can support up to six!) so that if they should fall, they have a phone on them to call for help.

There are still landlines in use (although they mostly use VOIP instead of analog phone lines). All cheap cordless phones I know run DECT. It's more than enough speed for speech and more stable than wifi.

The AVM Fritz product line of consumer modem/routers all include a DECT modem for registering extra handsets for voice but they have their own proprietary handsets that support TCP/IP over DECT for things like audio streaming or sending extra media like a caller photo.

I have a DECT headset and I would never go back to Bluetooth. The range, reliability and the quality of modern DECT is great. Just make sure you get one with ultra wide band or similar otherwise you end up with cordless phone quality.