Sending sms

To months ago I had a problem with my data contract, it keeps sending sms to the same number couple of times a day. The sim is however in a router and non was send. It cause my bill to go up and up and up. They fixed it, their words, only for 2 months and now it is back and worse than ever. Send them a query, when to outlet to no avail. Wonder what I need to do to stop it. Stop paying my bill will maybe get attention