This analysis doesn't concern the limited bandwidth available for call delivery on plain old telephone networks (POTS). They did squeeze extra money out of the system with their networks as a monopoly, but the cost was zero only if you don't consider the cost of operating and maintaining the network, or the opportunity cost of having much less bandwidth than currently available. For the former, they still had to fix problems. For the latter if they had made calls pennies everyone would have had "all circuits are busy" all the time. A single line wasn't capable of carrying 10,000 calls back then. Pricing to limit usage to available bandwidth was as important as recouping infrastructure costs and ongoing maintenance. There's also a lemonade stand pricing effect. If you charge too little you don't get enough to cover costs. But if you charge too much, not enough people will do business and you won't cover costs. Also, ma bell was broken up in 1982, but regional monopolies lasted a lot longer (telecommunications act of 1996).

TAT-7 which was in operation in 1985 when I cited the £2 per minute price carried 4000 simultaneous calls, ie up to £8000 per minute

Its successor TAT-8 carried ten times as many calls a few years later, industry professionals opined that there was likely no demand for so many transatlantic calls and so it would never be full. Less than two years later TAT-8 capacity maxed out and TAT-9 was already being planned.

Today lots of people have home Internet service significantly faster than all three of these transatlantic cables put together.