IMEI most likely. They can transmit the number as a non-reversible hash (MD5 or ...

		obviator on Dec 21, 2011 \| parent \| context \| favorite \| on: Over 700,000 Android devices activated each day IMEI most likely. They can transmit the number as a non-reversible hash (MD5 or similar), that way the number never gets sent or stored as plain text.