I've managed to get my IP number, by writing a php page and getting the REMOTE_ADDR string.
$clientip = $_SERVER['REMOTE_ADDR'];
echo 'phone ip:'.$clientip;
what i would like to know is whether i could use this ip number to track this specific phone. i.e. as a login name/password. so if a user logs on to my server today and uploads some files, in a month's time, then can come back and get only the files they uploaded. or will their ip change, or is it shared by other mobile users?
First, the IP address that you are seeing from outside of the carrier network is that of a NAT gateway. If you asked the phone for its address, by using eg Connection.getLocalAddress() you would get a different address in one of the non-routable segments of IP space (10.x most likely).
Second, a phone gets a new "networking context" (rather like a DHCP lease) each time it connects to the network, so if the phone is idle or turned off, it will likely be given a different internal address when it is next turned on, which may well map to a different external (gateway) address.
In summary, many phones will have the same external IP address, and a given phone will change both internal and external address if it is turned off, takes a long voice call, or is idle for more than a few minutes. IP address is useless for your purposes.