BT handsets locking Zen 7530 VOIP users out

Hi all. My first post on the forum.

For some years now I have used a quad set of BT “Advance Phone” handsets and base. My ISP and land line provider, Zen, made me an offer to change my ‘phone to their Digital Voice service. An offer I accepted. My fibre connection is FTTP Full Fibre 900. It all looked so easy, once the VOIP service was activated just register the handsets to the Fritxbox. I did. However I found that although they appeared in the interface on my Linux Mint computer I was unable to make or receive calls. After a lot of telephone calls, several hours worth in fact, Zen technical support were unable to offer an explanation. I bought a Gigaset handset and that works fine. If I connect the BT base into the Fritzbox Fon socket all handsets work as usual but this was not the way I wanted to go.

A lot of reading on the internet provided me with a clue. It seems possible that BT actively stop handset owners from using another provider. Please see these links.

Yealink locked

ThinkBroadband post

If my handsets are locked as those posts seem to suggest is any way of unlocking them?

In my experience it is the base station locked to BT, and it isn't so much locked, but set up to automatically configure itself to a BT number so it is plug and play.

The problem is likely that the handset isn't 100% GAP compatible, so doesn't register correctly with base station in the Fritxbox. If it is a Yealink handset they do say this in their datasheets

Yealink DECT technology is based on CATiq2.0, which focuses on high-quality Audio VoIP
(wideband), and low bit-rate data applications.
However, we are not compatible with any thirdparty DECT devices (base station, handset,
etc.).