Old telephone person here, who started his working life at Western Electric.
When AT&T bought up local telcos at the beginning of the 20th century, there were some that they never got to. GTE owned some of them, and some were independent. There was a Rochester telco, IIRC.
IT&T might have been involved with telephony at one time, but by the time of the divestiture they were a conglomerate with little or no telephone content. GTE had a bit more, and manufactured equipment. They actually had a research center in Waltham Mass. as of the early '80s.
The operating companies (and AT&T Long Lines, I suppose) were theoretically free to buy from anyone, but that would not have gone over too well. Western had a culture of absolute quality and reliability. Western was very limited in selling outside of AT&T, due to the consent decree from the '50s.
And everything was done with one eye on the regulators. I took a tour of the Long Distance control room in Bedminster, which used to be a famous scene. It was built to be just fancy enough to impress the regulators, but not so fancy as to make them think money was being wasted.
The OpCos were given a certain rate of return on their capital base. I worked at a research center (the only one in Western Electric) whose goal was to make manufacturing more efficient. It was paradise, since our budget went into the price of equipment, which jacked up capital costs and increased the profit of the OpCos, but regulators could hardly object to Western spending on ways of making equipment cheaper. Once a year the board of Western Electric came to hear what we were doing. They listened to talks in the morning, signed off on the budget, and played golf in the afternoon.
The old Bell System was an interesting place to work for.