I phone screen locks during calls and all the buttons stop working?

Try a forced restart of your unidentified iPhone model, using these instructions:--> Force restart iPhone

If the issue persists, it might be a hardware (proximity sensor? other internal component?) failure but we, just users like you, cannot diagnose that issue remotely, sorry.

I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

Your phone has a proximity detector that turns off the screen when you hold the phone to your face, so your face doesn’t activate any “buttons”. It is located at the top of the screen, and appears to be a tiny red “point” (smaller than a “dot”) that may be visible in the dark. If the proximity detector is covered by your hand, case or screen protector the it will result in the screen turning off when on a call, just as if you were holding the phone to your face.

If you are sure it is not blocked in any way you may have a hardware problem; contact Apple Support using the “Support” link at the very top of this page to discuss repair options.