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Strange SIP over VPN issue

Greetings, Our conpany has two remote sites which are connected with FG VPNs where using SIP phones to connect back to central office. For some reasons, SIP connections can only work one site at one time only. If one remote site is up, the other site' s SIP phones are all off and vice versa. And, it seems depends on which site' s VPN are up first. Othe traffic besides SIP are working find. Hope that there is an answer for it. We has been working on this issue for a while and with no luck.
New Contributor III

Hello, is it only SIP that causes the Problem ? Could it be that its a pure VPN problem? What Version are you running ? MR7 ? Using VPN in policy oder Interface mode ? -r.
New Contributor

Are you using DDNS for the remote sites? If yes, I had a very similar issue solved by having all remote clients registering with DynDNS instead of previously both NO-IP and DynDNS. However, the issue was purely VPN side - not SIP. Best regards, Mladen
Not applicable

Thanks for all your help. The connections are fine. Connections of VPN are working except SIP. We are using ver3 MR6. The remote VPN was connected using fortigate ipsec dialup (not DDNS). Lately I found out that it was the setting of SIP helper and SIP IP nat trace. For default, it seems limits the connections for remote source IP. Here is the link we use to solve the problem: and the command... config system settings set sip-helper disable end config system settings set sip-nat-trace disable end
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