At some point (perhaps after an upgrade), an iPhone has no longer been able to call or send SMS to another number with the help of an iPhone. a public telephone network. The functionality of the data was correct: it concerned only the composition via the local telephone company. The calling phone had no problem calling another number: the problem was limited to a specific number.
The receiving device was also an iphone. When the SIM cards were exchanged between the iphones, all the features came back. that is to say that one or the other of the phones can call the other,
How to solve this problem?