An easy way to rule out 02 or your sim card is to as a process of elimination insert your sim card in another phone and send a text.
Has both parties recently updated to Samsung One Ui / Pie ?
If everything is fine via using another phone then you can try processes such as a back up and factory reset.
A Samsung Experience Store or Samsung Service Centre can run a diagnostic test on the phone.
Daily Driver > Samsung Galaxy s²³ Ultra 512Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
