Agha,
You say that you are using the A800 as a gatekeeper with a private Ip, but you are trying to get calls from a phone with a public IP. Unfortunately, this will not work for several reasons. 1. The A800 will require both a public and a private Ip to allow the registrations between both and 2. when the external unit registers to the A800 and requests a destination for the number dialed, the only Ip addresses that the A800 has is the private IPs of the devices. The external (public) unit will not be able to route to a private IP. My suggestion for this is to purchase a call relay from Quintum (or reseller). The call relay would be setup on your private network with a public IP as well and you can have the external device route calls directly to the call relay and it will translate the public ip to private to your phones on the internal network. This is an IP-to-IP call.