FRITZ!Box 7581 Service - Knowledge Base
Making Internet calls over FRITZ!Box's VPN connection
If the FRITZ!Box is connected to a remote network via VPN, then the SIP accounts (Internet telephone numbers) entered in the FRITZ!Box can login to a SIP server in the remote network over the VPN connection. It is then possible to conduct Internet calls over the VPN connection to the SIP server. However, we cannot guarantee that this function will work.
Important:Enter the IP address of the SIP registrar and not its host name when you enter the account information for the Internet telephone number in the FRITZ!Box. The IP address must be from the VPN target network.