In my opinion, phone support is frequently requested but hardly ever needed. I worked at Twilio and as part of my role as a Developer Evangelist I responded to help desk tickets part of the week. We didn't do any phone support and we were able to help people effectively.
The key is to be very detailed, thorough and courteous in all your correspondence and also to ask for clarification if you don't understand their questions. You should also have a library of online resources like documentation, frequently asked questions and forums that you can refer people to so they can easily find answers for themselves once you point them in the right direction.
One caveat to all that though: I worked at another company that provided phone support but it was something you had to schedule and pay for in advance. At first, I thought it was crazy but our phone support people were busy all day helping customers. It wasn't a lot and it didn't even cover our costs but it prevented your phone from ringing off the hook and people calling to ask really stupid questions.
Hope that helps! Also, if you're looking for technical advice on setting up phone support options I'd be glad to help you... over the phone! :)