See my post
here. If your SIP client has a setting to manually set the external IP address (for use in SIP/SDP packets) it would be worth a try to set that to Voxalot's RTP proxy which is 64.34.173.199.
I am not sure if Voxalot is already trying to do this, but it seems to me that when a user sets Symmetric NAT Handling to 'Yes' Voxalot should look into SIP packets coming from the user, find all instances of user's LAN and public IP addresses, and replace them with the appropriate Voxalot IP address.