so i'm a little confused on your comment on the Blackview A70 phone. this sounds like a cheap off brand phone.

as for your samsung A70 have you tried rebooting the phone? maybe clear cache for the phone app as well. if all else fails you can try and do a factory reset.

 

1. Remove any accessories.
   Third-party protective films, cases, stickers, or glass screen protectors may block the sensor and should be removed. It is recommended to use Samsung-approved accessories only.
2. Restart the phone.
   Once you’re off the call, restart the phone by swiping down from the top of the phone with two fingers to open Quick settings. Tap the Power icon. Tap Restart, and then tap Restart again.
3. Clean the sensor with a soft cloth. the proximity sensor is located at the top of the phone, although the exact position will vary by model

I hope this will help