This was opened by Darren Taylor on the Microsoft Graph UserVoice site and it did gather 19 votes there as can be seen https://web.archive.org/web/20201205153559/https://microsoftgraph.uservoice.com/...
This would be very much needed for us. We have several customers which are interested in the question to whom they have most contact which is currently not possible. As more and more people are using Teams, the data gets worse. You can track normal Phone Numbers, but not external Teams Users. As you call more and more users over Teams and not by their Phone Number, you get less information. This is does not make sense to me and limits the scope of the API a lot.