[rxbudian]

It's probably to prevent multiple phones using the same number. Some network infrastructures are quite old and supports only the basic protocols. Even inside a single Telco company, the hardware is most likely very diverse. That means any new technologies must be backwards compatible to allow the new phones to use the old infrastructure. Checking whether a phone number has already been 'logged in' in another Telecommunication company's network takes a lot of coordination, and it has to be able to do that globally, in a very short time (a few seconds at the most). Then they have to deal with what should be done if the legitimate phone owner is the one that could not log in (Ie, someone actually used your number somewhere else) etc...etc... it's opening a big can of worms to get this going.