Hmm. So I could potentially do something like this:

https://www.amazon.com/Grandstream-Long-range-Station-5-UNITS-Handset/dp/B01LT6WFD8

Five handsets on the same DECT system, so call handoff would presumably work just fine, plus a base station that talks VoIP. This particular solution appears to have two separate base-station boxes -- one to speak VoIP and the other to speak DECT -- which seems like something that should be available in a single box from somebody. Or is it better to keep these distinct? (With the presumable interface between them being an analog old-school phone wire.)