ATT/Bell Labs invented the cellphone network...essentially the death knell for the landline phone business that it also created. They are also the fundamental creator of the network infrastructure for VOIP which end up killing the long distance business of the founding ATT.
Now, there are some complications because of the telecom business spinning off and buying each others parts...but the core has remained the same...and they are busy inventing the things that will continue to kill the legacy telephone business.