Slamming is line hijacking
Slamming or line hijacking is the conversion of a telephone connection to another provider without your explicit permission. Another phone provider may not accept a subscription without permission, telephone providers have made arrangements to prevent this. If you get an offer by phone for another contract, beware because saying 1 time is enough to be stuck (a signature is not required). Most people only find that they have become victims of slamming when they receive the new invoice or a farewell letter from the old provider. Consumers have been better protected since 2014 because there is also a need for written confirmation, for business users that is not (yet) the case. It is not only annoying if you are suffering from slamming, by transferring fines or legal fees it can also be a decent cost item. So ask if someone is calling on behalf of a particular party or really as an employee of a telecom provider, and use the words Yes or no not. If you have become a victim, please contact the (new) provider as soon as possible and indicate that you do not want to call this provider and that there has been slamming. An American movie about slamming, in the Netherlands there are of course other rules and practices, but principle is the same. Https://www.youtube.com/watch?v=lTkoOBim8Zk