Cloud VOIP issues with Sophos XG

Hi Guys,

I need some assistance understanding issues we are experiencing with a cloud hosted VOIP provider. The VOIP provider is not being much help.

We have an office with two XG230s in active passive, the issues always seem to be reported after any major network maintenance and then issues just die of over a few days (or people stop complaining). With network maintenance I mean anything that would restart the switches or firewalls.

The issues people report is a mix of the following issues infrequently

no dial tone when picking up the phone.
When calling from external calls go to voicemail even though no one is one the phone.
Sometimes it seems that extensions are mixed up, ie. I`ll call person Xs number but I`ll reach person Y.
Calls getting dropped completely.
At times they report voice quality issues as well but I do not believe its related to the above, and some calls just work.

There are Yealink(newer) and Snom(older) phones on the floor, the Snoms seem to have the most issues.

We have implemented DSCP marking (46) on the FW rule for VOIP traffic, added a VOIP Guarantee and also configured traffic shaping.

H323 and SIP module has been unloaded.
UDP timeout Stream has been set to 150.

Any ideas on further troubleshooting this issue?