iPhone sending messages by itself

I recently discovered my iPhone is sending text messages without any apparent interaction by me. Further investigation on reveals this is quite common on iPhones here in Poland. All the messages go to +4880473 and the seem to have no content.

Some people have suggested it's something on the network, but I found them being sent when I was in France two weeks ago so that seems unlikely.

Another suggestion is that an app is sending the messages. That seems unlikely as it looks like a major breach of privacy because I can't find anywhere an option to remove permission for apps to send messages.

Does anyone know of a way to stop messages being sent to a number or to trace what is sending them?

Blocking the number doesn't appear to stop outgoing messages. This is on iOS 15.4.