VOIp / watchgurd IP500 ?

Hello, looking for some feed back on best way of tackling/looking at a situation.  IP500 Avaya behind a WG xtm520 bovpn s2s to another loc(s) behind XTM505.  9620L VOIP phones at remotes(where ip500 is not). Users say will reboot and can see on display screen where phone is rebooting looking for IP. When on phone calls will drop and can see when rebooting.  No "jitter" is being reported as in "you're breaking up" "can't hear you" just phone rebooting throughout day.  I am thinking this is within the LAN as DHCP is local to the LAN but going back to the IP500 over tunnel at primary site, but I may be missing the best way to think about this/behavior.  

I've applied Qos on the voip policy and Traffic mgmt. as of last night on bovpn in and out policy preference five.  Wondering if I'm not looking at this issue correctly or what else I can try? Other input on troubleshooting.

No Avaya support but considering opening a case or looking for a local consultant but these things never have definitive answers, while I like to fix issues myself to understand and know it's done right.  Finally word has it AVAYA's official stance is that HW VPN tunnels aren't supported is their disclaimer while resellers do sell it b/c not all can have MPLS and MetroE?  

Hoping for some feedback/ideas.  

Thank You.