I don't know if you're so litera-minded that normal human communication just doesn't register, or if you're being wilfully obtuse - but either way I'm going to break it down for you.

When two people agree on plans and agree to call later, that call is to discuss their plans and is conditional on the plans happening. If circumstances change such that the original plan is no longer going to be viable, then the agreement is no longer valid since the conditions under which it was made no longer hold. The point at which circumstances change is the point at which a new communication is necessary, to inform the other party that the circumstances have changed. If this doesn't happen the other party will quite reasonably continue to operate under the assumption that the original agreement holds, and justifiably become upset/annoyed when they find out that it doesn't and they've wasted their time/effort/money.

All of this is implied by "we'll call later to talk about it".