From trawling various sources on the web it seems that as a foreign visitor you can use your phone for up to 120 days without registering it, using either a Turkish SIM, or an foreign SIM. If your phone were then to be blocked permanently thereafter this would be a big problem for many Europeans.
However, TurkeyResidentVisa.com has this to say about missing the registration deadline:
If you miss the due date for your mobile phone registration in Turkey,
you will have to leave and re-enter the country to register your
This implies that you should be able to use your phone on each of successive short visits without a problem. The article doesn't indicate what 'leave and re-enter the country' entails. Simply crossing the border into, say, Greece for 20 minutes might not be enough.
Disclaimer: I have no idea how authoritative this web site is. It's the only place where I found mention of anything like the 120-day problem for visitors