VOIP doesnt work correct over IPSec

Hello

We have two offices. One is the main office and the other one is a side office. In the main office we have a FortiGate 60D and there is as well the VOIP server(Swyx). Our side office has an Edge Router Pro. These two routers have an IPSec tunnel.

The problem we have is, that the VOIP communication between users from the side office and users from the main office is not working. Most of the time the two users cant hear each other. The strange thing is, that sometimes it works. Its also that for exampel one user from the side office cant hear one explicit user from the main office, but if another user from the side office calls this explicit user from the main office, they can talk to each other. It is really unpredictable.

Here is the IPSec config from the FortiGate:

config vpn ipsec phase1-interface
    edit "SG"
        set interface "wan1"
        set nattraversal disable
        set keylife 28800
        set proposal aes256-sha512
        set dpd disable
        set dhgrp 16
        set remote-gw PUBLIC-IP
        set psksecret dfjsvdsl
    next
end
config vpn ipsec phase2-interface
    edit "SG"
        set phase1name "SG"
        set proposal aes256-sha1
        set dhgrp 16
        set keylifeseconds 3600
        set src-subnet 172.200.1.0 255.255.255.0
        set dst-subnet 172.190.1.0 255.255.255.0
    next
end

And here is the firewall config:

config firewall policy
    edit 17
        set uuid 05e77718-20b8-51e5-fca6-956d779eb92f
        set srcintf "SRC"
        set dstintf "IPSEC"
        set srcaddr "172....."
        set dstaddr "172....."
        set action accept
        set schedule "always"
        set service "RDP" "SMB" "ALL_ICMP" "VNC" "SIP" "Outlook Messenger LAN" "Swyx Anmeldung am Server" "DNS" "HTTPS" "HTTP" "Swyx! CallControl" "Swyx! Audio" "SSH" "iperf"
        set logtraffic all
    next
end
config firewall policy
    edit 15
        set uuid f40a56c8-20b7-51e5-a4b5-a239a77c555a
        set srcintf "IPSEC"
        set dstintf "SRC"
        set srcaddr "172....."
        set dstaddr "172....."
        set action accept
        set schedule "always"
        set service "RDP" "SMB" "ALL_ICMP" "VNC" "SIP" "Outlook Messenger LAN" "Swyx Anmeldung am Server" "DNS" "HTTPS" "HTTP" "Swyx! CallControl" "Swyx! Audio" "SSH" "iperf"
    next
end

Do you have any idea, where the issue could be? Do I need the Traffic Shaper and set the priority to high?

I have posted a similar question in the UBNT forum, where I am hoping to get some tips for the Edge router and here I am hoping to get some inputs for my FortiGate config.

Kind regards

Joel