I want to be able to collect information about mobile phones that access my WAP pages!

I also want to uniquely identify when a phone revisits my WAP site so need to log something that is unique about each phone.

Here's the code I've got so far:

echo("REMOTE_ADDR: ".$REMOTE_ADDR."<br/>\n");
echo("REMOTE_PORT: ".$REMOTE_PORT."<br/>\n");
echo("USER_AGENT: ".$HTTP_USER_AGENT."<br/>\n");
echo("HTTP_VIA: ".$HTTP_VIA."<br/>\n");

echo("Subscriber ID: ".$HTTP_X_UP_SUBNO."<br/>\n");
//this only works in USA apparently and the site is for a global audience so this isn't much good!

Has anybody managed to uniquely identify devices? It isn't possible to get the phone number from the WAP gateway, so that's not an option? Is it?