RED behind Cisco firewall router: SIP Problems

Hi all!

I have some problems with a RED15 in a home office. This home office have a cable-internet-connection with a cable modem (without any firewall). The comes a cisco RV345 firewall-router. Than comes the Sophos RED15.

The tunnel to the main office comes up and file transfer and RDP throug the tunnel is possible. The SIP Phone connects throug the tunnel successfully too. But the phone can not start or receive a call.

Same Problem when replacing the cisco with a FritzBox.

If the RED is directly connected to the Cable-modem (set to routing mode) the SIP Phone can do calls through the RED Tunnel.

So i know you will tell me to place the question in a cisco forum. But i need some details about the RED Tunnel first:

- What Ports are needed by the RED Device?

- Do i need to allow or to do something on the Firewall / Router in the home office side?

- Do i need to allow WAN Pings?

- Is there somethin i need to turn off on the router?

Best regards!