The “extip” field will not help you on this scenario.
According to Quintum description, this field is used to modify the H323 packets sent out, to “fake out” the I.P. of the gateway.
The billing system does not talk H323, only TCPIP.
The “extip” will only matter when you are passing H323 packets betwen two H323 gateways.
And even then, most people could not make it work properly.
Every gateway that I have seen with NAT , have the billing also in a internal I.P. address.