I agree. Three possibilities:

1. The stolen phones are yet to get a buyer :)

2. The stolen phones are now generating revenue for another operator other than the one you are monitoring on

3. God forbid, the IMEIs of the stolen phones have been tampered with (changed to something that is deemed valid to the local operators)

The show-stopper in containing handset theft is more operational/regulatory than technical.

So long as a handset blacklisted on one network can still be used on another regional network, we are in for a looong ride.

Simple collaboration between the regional OpCos (sharing of blacklisted IMEIs) will drastically cut down the market for stolen handsets. But then, it's all about the bottom line... not so?

PS : Am I misguided to think that the enforcement of the 'fake-phone-switch-off' was superficially done? My guess is that these OpCo's may simply be sniffing out for handsets that bear no IMEI as opposed to doing a comprehensive look-up of the IMEIs that latch onto their network. (GSMA has the full database)