Does this happen everywhere you go or in a particular area ? I' thinking it could be network mast issues if in a particular area !
As a process of elimination can you insert your sim card in another phone to rule out the phone or sim card that maybe at issue.
If your sim card is fine in another phone then are you able to try a friends etc sim card in your phone.
A Samsung service location or Samsung Experience Store can run a diagnostic on the phone if necessary.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.