Do I need to register my phone in Turkey if I'm using a UK SIM card?

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 phone.

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}

According to the prepaid SIM card wiki, which is of course unofficial but usually reliable, you do not have to complete the foreign-purchased-phone registration process if you are using a non-Turkish SIM card.

If you can read Turkish, you may be able to find more information from the MCKS website, though their English website does not appear to contain any relevant information and I suspect the problem is due to just being an unhelpful website rather than bad translation.

If you use foreign SIM card, you do not have to register. However, if you ever use a native SIM card, then 120 days period starts and whatever you do (leave-come again), after 120 days, the phone will be blocked for native SIM cards. After this point, could you still use a foreign SIM card again? There is no official information, but by experience, at least after leave-come trick, yes. However, with a native SIM card, until registering it, never.