Hi Dawn,
This is a great question - I can see how you'd be uncertain as to how to proceed.
I would assume that if the client provided both phone and email in their inquiry to you then you can get in touch with them in whatever way works best for you.
Personally, I prefer to communicate via email for the most part but do call new clients in order to help develop the relationship.
With that said - if this was my own circumstance to deal with I'd probably respond with a detailed email and close with an offer to phone them for further consultation.
Hope that helps! Surely others will post back with their own suggestions