Using 2820 IPPBX FW3.5.5.1_232201 connected via ISDN lines. Yealink t22p handsets.

We are having an issue where several calls a day are dropped a few minutes into the call.
It is quite difficult to find the cause of this issue, whether handset, network cables, switch, router or isdn line issue.
are there any tools available to help in tracking the cause of this issue?

I have set up syslog, all the sections in it are collecting data, except the IPBX section.
The call detail records page does not help, as does not say why the call ends, only that it successfully connected for x amount of minutes.

What tools are people using to help them diagnose issues?

I am ruling out handset, and cabling as possible causes as this has happened on multiple handsets. and i have performed tests on the network cabling. But i still need to work down to switch / router / line

This is the second 2820 ippbx on this network, the previous one was running latest firmware, but was rebooting itself every 2-3 minutes, so i am hesitant to update firmware on this router as at least the router is stable.

Any help in finding some methods of fault finding this would be greatly appriciated
Thanks in advance
Matt