T41P phones not reconnecting to 3CX

[3cxuser]

We have been having issues with Yealink phones, primarily T41P, not reconnecting to 3CX in event of connectivity loss i.e. internet router rebooted or ISP issue.

Initially we started with 3CX hosted through a 3CX partner, it was v14. We then purchased our own 3CX v15 PRO 8 calls and host it in Azure cloud. So we changed the hosting company, changed the 3CX version but the problem remains. We have an SSL Certificate and the CA is on the Yealink approved list. Phones run the latest firmware 36.80.0.130 and have been reprovisioned and reset a few times. The problem does not affect all phones and every time. There is no pattern on one occasion 2 out of 12 phones pop out, another day it could be a different phone etc. I have to reboot phones to connect to the network again.

It looks like the issue is with the phones rather than the phone system. I would appreciate any advice