SMS Anti Theft

I have not searched the forum so it is possible that the answer to my question is already there but still…

I’m using Avast Mobile security and anti-theft. When I’m connecting to a Wifi abroad it seems to send messages to the phone I put down as first friend. A lot, because I had a huge bill coming back from a trip abroad.
My provider was not able to see where the messages came from or to whom they were send but I still had to pay several hundred euro’s. Now I switched provider and the messages popped up on my usage app and also the number (first friend) came up. So now I’m asking myself “what the F…”.
OK to make something clear. I’m not looking for compensation but an explanation and possible a way to avoid this in the future. If some techguy can confirm this theory and tell me how to avoid this I would be grateful. Otherwise I’m throwing Avast of the phone to see if that is making a difference. And if so look for another product.

David

https://www.avast.com/faq.php?article=AVKB135
https://blog.avast.com/2014/07/02/how-to-prevent-unnecessary-costs-of-avast-sms-notifications-when-replacing-your-sim-card/

OK Nice explanation for the fact that they popped up now. I did change to another provider so yes clear but the other phone did not receive anything so how about that? And what about the trip abroad. No SIM card change just a lot of hidden messages every time I connected to the hotel Wifi. For location purposes? Any suggestions?