Vonage and dropped calls through SG135

Hello everyone,

We have had this issue for quite some time and support was unable to give us answers after spending days troubleshooting and reviewing logs live. Hoping that someone from the community has seen this before and can help.

About 25 users behind this unit, heavy phone users - mostly sales staff, experiencing call quality degrades from "robotalk" and mid word "exhales" to calls just dropping entirely.

So far we have:

-Added a separate dedicated cable uplink = WAN 100/10mb

-Vonage support (not much networking help) can not give us their server IPs or their AWS balancer for "security reasons" and we have seen phones register to different WAN IPs. Therefor, we can not make use of the SIP Support feature on UTM.

-Separated LAN traffic via VLANs

-Applied FW rules of: VoIP Network > ANY > Internet IPv4

-IPS (Anti-DoS/Flood and Anti-Port Scan) Global Settings: VoIP Network removed

-Advanced Treat Protection: VoIP Network is exempt

-Swapping out our UTM with a plain router (of different brand) clears up the issue so it is not on ISP side either

Anyone has notes that they can share on what it takes to get these UTMs to play nice with Vonage?

Thanks for your time!