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 wi

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.