Yes the IMEI can be surveilled to determine the nature of the device. That's one of the first pieces of data a network finds out to know if the device is allowed in; yet, the IMEI is not listed in a "friendly devices" ledger. In CDMA networks, the incorporation of IMEI is recent and only regulation-compliant, for what the network asks for is an account number and the ESN, that unlike the IMEI in the GSM networks, is in fact listed in a "friendly devices" ledger. That's why hacking or cloning numbers into CDMA networks is very difficult. Other means of detecting the nature of the device involve bandwith query; a device requesting OTA connection (GPRS, UMTS, EVDO, etc.) will ping requesting for a broad channel, and the network server has to reply to it, and may "ask in return for credentials". Or, routing footprint. A Blackberry device, AFAIK, will search for the RIM servers, whether BIS or BES services are activated on the device or not (that's one of the pieces in their push email magic). And I bet there are several other symptoms that an OTA connection is trying to get closed. Actually, a carrier that just closes any wireless gateway for PAYG accounts is being compassive. I've read of carriers that take the attempt as a mutually approved contract to switch into postpaid plans. Oh, the surprise that can be delivered at the end of the month by snail mail . rajdude, in a nutshell, this case study by Hook involves using a PAYG account as a bank account, or an electronic wallet, from which you do payments in packs for assorted services. I encourage you to read all the thread cause it's full of itty details that can turn important any given instant. And you may have to look for similar options with your carrier.