I know that, in most of the world, phone conversations are transmitted as digital data (whether frames in circuit switched protocols or packets). I know that several companies offer products which will convert GSM conversation data to VOIP at the base station or MSC level. I assume this data is not archived or saved anywhere (unless requested from authorities to the phone company). I think (but am not sure if this reflects today's world) this data is not encrypted for most of the travel from A to B (in GSM it's actually encrypted from the handset to the base station). I do know that most of the network infrastructure is reachable via IP from inside the owner company for management purpose.
My question is: is it technically possible for bad guys (or governments) to infiltrate from outside the routing infrastructure (without the phone company's knowledge) and archive / copy not just metadata, but conversations of one or more specific persons? Has this happened in the real world so far?