This is exactly the kind of comment I was hoping for. Thanks!
But, I'm unclear that this is very cost effective. For example, is this the kind of device you mean?
https://twenty7tech.com/product/pots-adapater/
That's $300 and then requires a SIM card as well, right? So, let's say I use a cheap one from Tello that is $6/mo.
That's still a lot more than using Plivo/Twilio with a SIP phone, right?
<$100-120 ATEL LTE on Amazon cheaper than that one. The adapters are such you can use a traditional legacy style oldphone.
DECT phone uses local RF (like cordless home phones, think of it as the "phone to headset via bluetooth" portion), a SIP phone uses the internet directly, and an LTE base station provides a fake bridge from POTS button presses to a mobile call dial.