Uploaded 12.5.7 onto my iPhone 6s and now I cannot make or receive phone calls

Did you perform a Forced Reset to give everything a fresh start after the update?

Force restart iPhone - Apple SupportOfficial Apple Support › guide › iphone › ios

Follow the instructions very carefully. Keep holding the button down if the "slide to turn off" screen appears until you see the Apple logo appear in the center of the screen.

Force restart iPhone - Apple SupportOfficial Apple Support › guide › iphone › ios

If you are still having issues after few tries, you will need to contact your carrier since they handle incoming and outgoing phone calls.

If you updated your phone to 12.5.7, then you have an iPhone 6 or 6plus, not an iPhone 6S

See if this helps -

Update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

If you have an iPhone 6s as you indicate, the latest update is iOS 15.7.3.

If you really have an iPhone 6, the latest update is iOS 12.5.7.