It could be the network connection to the available masts @Jay245
Or could be am issue from the other end i.e the persons your calling.
Can you as a process of elimination try your sim card in another phone and then see what happens.
And if possible try another sim card in your phone.
You can reset network connection in Settings, General Management, Reset and Reset Network connection.
You can also run a diagnostic test via your Samsung Members app.
A Samsung Service Centre can assess your phone.
If I can be of any further help then please don't hesitate. Take care. 😎
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. "