If you have a land line, odds are that your local phone, your long distance service, and your cell carrier are all carried by pieces of AT&T. Pieces that would have reassembled if not blocked by anti-trust. And even if your provider is actually new, they could not have come into existence without rules requiring interoperability.
Some monopolies are "natural", as in once one is established, it is hard to squeeze in on them. For example there is no point to a phone that can't call all the people you want to call. So everyone naturally gravitates to the same network. Most businesses are not natural monopolies. Just because my friends shop at Ralph's doesn't mean that I can't shop at Safeway.
Whether you have a natural monopoly isn't a question of size, it is a question of the unit economics. Retail is a notable example where your advantage as the 800 pound gorilla only goes so far. But technology has many examples where your advantage can be more lasting. (AT&T's decades long monopoly is the best example.)