[cisco-voip] Cisco 2811 problems when used to terminate vpns

[Manuel Marin Garcia]

I have the following scenario

IP phone
   |
Netopia Router
   |
Internet
   |
2811 with CME/VPN
   |

POTS lines

The VPN works fine for DATA, the phone register with call manager but 
when I try to make a call I see that the call is connected but there is 
not any audio. I know that I need to configure some kind of loopback 
interface but I do not how. Please does somebody already configured 
something similar to this. Please help me!