"Slamming is the unlawful practice of changing a subscriber's selection of a provider of telephone service without that subscriber's knowledge or permission."
I was forced to upgrade my POTS+DSL to fiber recently, with the threat of losing all service. I have had AT&T international long distance for 30+ years. After about a month I discovered that Verizon had hijacked all long distance, including international, leaving me with a huge bill because I now have no international calling plan with anyone (not even Verizon).
I called AT&T and they confirmed that they were no longer receiving my long distance calls.