Roaming with VoIP question

[guest]

Hi,
Could you please provide some detail on roaming with VoIP, assuming the ISP provides roaming and allows taking your ATA on the road such as going overseas? Specifically, how does the network know where in the world you are?
Does the local ISP gateway interrogate the ATA MAC address? Is your telephone number coded into the ATA and does the ATA transmit your telephone number to the ISP?
I can see that for outgoing calls, you provide your location, subscriber details and destination address. However, when you first plug in your ATA, how would an incoming call find you? Once you are on line at the
new location, the local ISP would assign an IP address to you, but how does it associate that IP with your telephone number? Is the DNS server involved?
Regards, George

[Tony]

It is done by MAC address authentication.

Tony