Apparently your E52 supports STUN, see
this page. Maybe STUN is not yet configured in your phone. Without STUN the E52 uses it's private IP address/port inside SIP packets, including the RTP contact address, the address/port to which the other party attempts to send the voice packets. If the VSP does not support SIP re-routing of RTP stream you will still hear the remote party because RTP packets come to you through Voxalot. It is good to use Voxalot's "Symmetric NAT Handling" because that routes voice packets through Voxalot only when necessary to complete the call.