You authorized it when you decided to use the reply to text option on the watch. The only way to disable it is to disable text replies. I believe this only happens on iOS but I could be wrong.
It should only happen once every 30 days. It only started for me when I started using my pebble on an iPhone 8. I had my pebble when I had android and it worked fine.
You gave it your phone number when you set it up, I believe.