My NEW S21 rings and sometimes the answer screen does not come up, so I can't answer, reject or send a message. I will reboot the phone, but how do I keep this from happening as I use my phone for business and it's not good to be perceived as ignoring calls?
My suggestion is to clear the phone's System Cache Files and make sure your phone's software is updated.
Clear your phone's System Cache Files by turning your phone Off and then from a set series of button presses go into the phone's System Files and clear the cache. No information is lost this way. You may need to insert your headphones or hook upto a computer with your data cable to get to this menu on the phone. https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/
Also as a process of elimination to rule out the phone or sim card can you try your sim card in another phone and if possible another sim card in your current phone.
In your Samsung Members App under GetHelp there is a Diagnostic Tool.
Ultimately a Samsung Experience Store or Samsung Service Centre can take a look at your phone's.
Current Phone > Samsung Z Fold³ 5G.