Use the telephone for outgoing requests, email for incoming requests
I have been thinking about emails vs phone, or more generally asynchronous vs synchronous communication, and have realized:
If you receive a call, it breaks your flow, which is bad.
If you send an email, it breaks your flow, which is bad. (As you have to wait for an answer, and do something else in the meantime.)
If you have a question, the optimal strategy (for you) is to make a call.
But if someone else has a question, the optimal strategy (for you) is they email you.
There's even a further level, which is that if someone else has a question, it's optimal (for you) that they email you, and you reply by calling them, and discuss it until it's done.
So if it's up to you (and you're thinking only about your own productivity above all else) then tell people to email you when they need something, but call them when you need something.
P.S. Here's a great blog post (German) for those who fear being called on the phone: Warum ich nicht mehr telefoniere