Core Issue
If a Cisco Softphone user has a Cisco VPN Client installed with the stateful firewall option checked, then the Softphone user will not hear the voice of the user on the other side. However, the user on the other side hears the Softphone user.
This issue affects any Softphone call initiated or received, even if the VPN is not being used.
Resolution
Uncheck stateful firewall (always on) under Options in the VPN Client.
This issue has also been experienced on third party VPN Clients.
For more information, refer to Cisco VPN Client.