This is a recent decision.  They are more paranoid about you going to
the competitors.  As they have a few international contracts they want
you going through them rather than finding some-one cheaper.

Yazzan Gable <gabley@titans.org> wrote in article
<gabley-F42BBD.18251322092003@news.supernews.com>:

Apparently.

I got an e-mail from someone who tried using the e-mail address
for SIM unlocking by Cingular. He got a reply back that he had
to call customer service for unlocking, and customer service
refused to unlock it.

I suspect that the new policy has something to do with WLNP,
and the fact that enforcing the terms of a contract vis-a-vis
termination fees, cannot be used to stop a customer from taking
their number with them. With or without termination fees, Cingular
is worried about more subscribers taking their phone with them to
another carrier, and sees locking as a means to stop subscribers
from leaving.

AT&T already had a policy of not unlocking phones, not sure if
T-Mobile still will do it.

Of course you can still buy an unlocked GSM phone at full retail
price and use it with any GSM carrier that you desire.