Wikipedia1 defines Telephone Slamming as
an illegal telecommunications practice, in which a subscriber’s telephone service is changed without their consent.
Slamming can be annoying and time consuming for a consumer to sort out and may lead to higher charges and being locked into a contract that they don’t want to be in. For businesses, it can be much worse as if a company’s phone service is slammed, the scammer could change contact details so that all notifications go to him and then he is free to re-direct calls, change outgoing messages and intercept voicemails. This would allow him to intercept customers who are trying to contact the real company and either steal the business or just outright scam them. The first that the company might know about it is when they start getting complaints or bad press.