It's been know those "free" conference call numbers actually charge the provider for it. T-Mobile said that's enough and stopped paying those fees. Other carriers are not "out of plan." I don't know where you got that information from.
TMobile recently did start charging their customers for these types of calls. They are not the only carrier that does this. The problem is that these services are not free. They charge the carrier a termination fee when you call one of those numbers. This is the carrier passing that charge onto the consumer.
Ok here's what is going on and what they are trying to stop. Traffic pumping
AT&T actually started blocking people from connecting to these numbers and continue to add to the list. There is a legal battle going on over it.
Verizon has been rumored to start charging or cutting off depending on the outcome of the verdicts.