A couple of websites I found that deal with the theory of IMEI registration:
This one is a Q&A in English:
Avea: Unregistered devices
This government site has an English bit about "Temporarily Staying In The Country" which implies that a registered phone can only be used with a single number issued by a Turkish operator. (This seems to be different from an 'Individual Import')
imei.tk.gov.tr
It also allows you to check whether an IMEI is registered (IMEI Sorgulama). The link above that seems to check whether an IMEI matches a MSISDN.
All that is
theory of course; the
practice may be quite different
