It would be good to identify where the static is coming from.
If there is static when the phone is used by someone else, then we’d need to eliminate sources of interference from the phone. Since you used at least one Dect 6.0 phone, though, I’d assume the phone is OK. These are hard to interfere with.
I wear Phonak microSavia Arts, and have a Uniden DECT3080 phone. This phone is Dect 6.0, but I found it did not activate my Tcoil. I either had to use my remote control to force Tcoil, or put an accessory magnet on the phone so that when I put the phone to my ear the aids switched to Tcoil. (As an aside, now that feedback is correctly controlled, I don’t need Tcoil for these phones.)
But regardless of Tcoil, I didn’t get static or buzzing, I got whistling feedback.
So I thinking there is one thing that always gives me static or buzzing, and that’s when I get close to a magnetic field. Doorbell speaker systems, some UPS power supplies for computers, some devices that eg. dim lights . . . these are examples of stuff that sets up a magnetic field, kicks in my Tcoil, and “gives me a buzz” when I get close to them.
Could any of those be the culprit?