We are running a Sophos XG Firewall and S705 Phones in this setup. I also have another setup with S705 phones but not XG.
When calling in or dialing on the phone pad itself they are “static-y” when dialing or ringing. This has happened with both of these deployments. I have added an S700 to the mix and its doing it as well. They are all on the latest firmware for the phones.
Once connected the call quality is pretty much just fine. No issues there. These are up-to-date Cloud hosted PBX’s in the CyberLync Datacenter.