Does this help? Found it on Cisco's website:

https://www.cisco.com/en/US/products/sw/voicesw/ps1860/products_tech_note09186a0080094ed1.shtml

One-way Audio on a VPN Connection from SoftPhone

When you configure a SoftPhone to connect through a VPN, it is important for Cisco CallManager to receive correct information about the IP address the SoftPhone uses. By default, the Cisco IP SoftPhone uses its physical network address to register with Cisco CallManager rather than with the VPN address. Cisco CallManager instructs other IP phones to send to this address. The result is a one-way voice since the IP phones are able to reach the VPN address of the PC and not the physical interface of the PC.