said by priller :

Your VoIP provider's SIP proxy knows the address and port you registered from, so it just sends the INVITE to you on that port.

In my example above, the INVITE comes to me on 1061 ... is translated back to 5060 and is passed along to the ATA.

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Please don't screw with port forwarding. If for some reason you have a problem receiving calls, the problem is that the NAT translation in your router is timing out. This is resolved by adjusting the timeout in your router. If that isn't an option, then request that your provider increase the registration interval or enable NAT keepalive on the ATA.