You are starting with a presumption that the first company's infrastructure just appeared out of thin air and cost nothing?
Let me spell it out for you:
First company comes by and installs the line, charges money, prices slowly creep up on the natural monopoly
A few years later second company wants to provide similar service, but perhaps targeted at more/better/different population or wants to do it cheaper
Option 1: The two parties come to an agreement to rent the infrastructure at or below the cost that it would take for party 2 to implement new
Option 2: They don't come to an agreement and party 2 now implements a second line, but because the advancement in technology and lessons learned from party 1, can do it for a lot cheaper. There are now two lines, but party 1 goes bankrupt as everyone goes for the cheaper option (or party 2 goes bankrupt because they underestimated the true cost of running business)
At that point, the assets of party 1 can be purchased by party 3, party 2 can decide to make the same mistake as party 1 and die or actually compete.
That's how it works in a non-government controlled market. Every single free market operates this way, it works for grocery stores (average profits under 2%) it works for delivery services (average profits under 1%). If a company is overcharging or not delivering services, people vote with their wallets.