Client VPN voice problem

Hello,

I have the following problem. Our users use FortiClient at home. Via a client VPN tunnel they are connected to our office. There is a FortiGate 500D (FortiOS 5.4.6) in use. We use Avaya Agent Desktop (Softphone) for telephony. 3-4 calls go through and you can understand the other one. But at some point it rings, but the conversation is not transferred. I see in the firewall logs "only" an IP connection error to the appropriate server. After what I've read about IP Connection Error, this problem has nothing to do with the firewall, but an endpoint, be it the client or the telephony server, are not reachable. Interestingly, a connection is still established relatively at the same time, see diagram. Does anyone know where to look to solve the problem? SIP Helper is not active on our firewall. Thank you in advance for any hint.