no the Phone Call has nothing to do with the VPN tunnels.
The Phone Call goes direct to my Internet Provider (German Telekom). The phone devices are in a own subnet (telephony only) and the Policy allows only connections from the VOIP-subnet to the VOIP Provider over WAN.
Sometimes the phone calls are disconnected without any configuration change, but when i make changes on policys or VPN connections the call was disconnected every time.
With the UTM 9.x all works fine. so i think the voip configuration is ok.