I'm a foreigner who is going to live in Turkey for 9 months. After two weeks here I bought a SIM card and I received the usual messages about the registration. If I remove the SIM and put it in a phone I purchased here, will my previous phone get blocked or will I still be able to use it with no card?
SIM stands for Subscriber Identity Module. It identifies the phone it is in to the cellular network. When removed from the previous phone, that phone will no longer register itself to that network and so will not be able to receive calls, text or data for the number with the SIM.
The phone itself will still work and you are free to put back your old SIM in it which will connect in roaming mode so that you can communicate on the old number which is generally costly due to roaming fees but those depend on the plan of the old SIM.
Not inserting any SIM in the old phone still leaves it as a functional device which will be able to connect to WiFi, use Apps that don't require the SIM, watch videos, play music, etc. You can even install some apps that allow to make and receive VOIP calls and text while you are connected to a WiFi network.
All modern phones should work without a SIM card. A very long time ago, over 10 years, some models refused to access any functions until a SIM was placed. Even so, it was a simple task to edit the firmware to disable the check since I wanted to give the phone to my young daughter to play with.
If you used a turkish SIM card in your phone, the phone's IMEI will be blocked in Turkey after a certain time unless it is registered.
In this thread on tripadvisor https://www.tripadvisor.co.nz/ShowTopic-g293974-i368-k11432822-I_used_my_mobile_in_2017_in_Turkey_will_it_be_blocked_2018-Istanbul.html A user claims, that using the phone in your home country with a local SIM will reset the IMEI ban in Turkey. But I haven't found any other information to confirm this.