Phone calls are just terrible when you try to convey complex information. Just recently had that issue with a business deal where it only became clear that we were not on the same page w.r.t. some important details after I put things into writing after an IRL meeting and several follow-up calls. The lesson from this is always to follow up with a summary e-mail, and to avoid anything more complex than a simple yes/no or maybe arranging a time and date by telephone.