Call them right after you send the mail.
At the beginning tell them that you sent a mail about the issue, but you want to ask them about it. There is nothing preventing you from calling since you sent an email. The mail creates a papertrail, and gives them a heads up about before the call, or allows them to look at it while taking the call if they choose to do so, but then you get the benefit of taking care of the issue quickly on the phone.
Rather than asking them to respond to you, you just just open a medium of communication that gives a natural quick response, so there is no need for them to get upset by you continuing asking them for a response.
I do this almost every day at work -- I get to speak about it normally as if I had called first, and I've never had the other party complain or get upset about it. In fact, some of my contacts have started doing the same to me.
