I'd say a phone call as well. It makes sure that the correct person gets the information and gives some sort of response. You don't get on the phone and expect them to give you their answer in that call. You only expect them to tell you that they will speed up their evaluation of you and possibly give you a firm committment of when they will have made their decision.
The problem I have with phone call is that not only do you put them on the spot, they can turn and put you on the spot if the give you an offer.
What's wrong with email? Is it really inappropriate?
An e-mail isn't inappropriate. It just isn't as quick. If they don't write back for a few days, you are left wondering if they got it and are working on it, are just ignoring it, or if it got caught by a spam filter.
Sure it is possible that they put you on the spot during a phone call. I find it unlikely. Any reputable place knows you need more than one minute to make a decision.
Doing it with a phone call just tells them that it is an important issue. E-mail communications are less intrusive, but it also tends to say that you can wait a day or two before hearing their thoughts on the issue.