Funny, I was just speaking about this with a coworker this afternoon. One of our office staff is notorious for sending message after message after message about things that, as you said, could be cleared up with a 30 second call. It's even more frustrating when this person does that even after being instructed to call the customer/vendor/whoever.
As far as when I do which, it depends on the content and priority- if that makes sense.
Some instances where I just email:
- Questions for coworkers that I don't urgently need answers to.
- PO requests (I even customized a spreadsheet that has complete info so there's no question about what I'm requesting) involving "normal" delivery. If I need something urgent, I still email and follow up with a phone.
- If it's something that I can wait on getting an answer to, I'll email.
- It I'm just following up.
Instances where I call first:
- Questions needing an immediate answer (and, if the person doesn't have an immediate answer, a follow up email gets sent as a reminder).
- Urgent PO requests (again, with the follow up).
- Any instance where it would take longer to type up the email and send it than just dialing.
For all the good of being able to communicate easily, have a record of said communication, etc.- email has ruined (my opinion, anyway) many people's personal ability to communicate.