The phone technically doesn't solve the problem, because phone connections can be flaky. The equivalent problem would be: you call the other party and say "I will attack at 9 am if you agree" and then you get a disconnection. Then to 'prevent that' you want an OK, but to get certainty (which is absurd in this case because of the latency and the low chance of failure) you should OK that OK. Repeat infinitely.

There is one feature of phones that sort-of makes a phone more useful here. You can know whether your 'message' arrived. Because if the other side hung-up then you get a dial tone.

The line could be intercepted, and have you get the dial tone instead of their OK. They would attack without you.

It doesn't, not really. It ultimately just creates an illusion of agreement due to faster transmission.

Right - the data are still being sent in packets, but the quantity and frequency of the data packets are such that the chance of miscommunication is reduced to near zero with even a modestly strong connection.

At the risk of being oblivious to humor:

The point is the latency and loss of the message -- for a more red-team angle, suppose someone in the intervening space has captured the communicator and seized their phone?

The story is an allegory (metaphor?) for the inherent unreliability of communications between any given pieces of real-world infrastructure and the problems that arise when that unreliability reaches critical thresholds.