The IMEI rule, and who it actually affects
Turkey operates a device registration system. A foreign handset used on a Turkish network is permitted for a limited window — commonly cited as around 120 days, though the figure has changed over the years — after which the IMEI is blocked unless registered and a fee paid.
The important clarification: this applies to devices connecting to Turkish networks over an extended stay, not to short-trip tourist data plans. A travel eSIM used for a normal holiday falls outside it, and there is nothing to register or declare.
It becomes relevant if you are in Turkey for months, or if you visit repeatedly on the same handset. Anyone in that position should check the current rules directly rather than relying on a summary, because both the window and the fee have been revised more than once.