Software VPN clients are overlaid on top of an existing IP network, meaning that there are essentially two IP addresses on the computer when a VPN is in use:

"The IP address from the underlying network

"The IP address provided by the VPN client that is used by parties on the remote side of the connection to communicate with applications on the computer

Some VPN clients, such as Cisco Systems VPN Client 3.x, assign the VPN IP address at a very low level, which makes it difficult for Cisco IP Communicator to specify the correct address. To eliminate this problem, Cisco IP Communicator queries the Cisco VPN client directly.

Other VPN Clients, such as the Microsoft PPTP client and Cisco VPN Client 4.x appear as alternative network interfaces. In these cases, the IP address can be selected with the same auto-detection process that is used to resolve selection when there are multiple interfaces.

Other third-party VPN clients might be unsupported and result in one-way audio. You can resolve this problem by running the Cisco IP Communicator Administration Tool to create a getIP.asp audio IP address reflector web page as described in the "Resolving Audio IP Address Auto-Detection Problems" section. Cisco IP Communicator attempts to fetch this reflector page rather than using other methods of auto-detection. The reflector page returns the IP address from which it sees the request originate, which is a relatively reliable way to identify VPN IP address of Cisco IP Communicator.

http://www.cisco.com/en/US/docs/voice_ip_comm/cipc/2_1/english/administration/guide/cag21cfg.html#wp1045281