If the support staff knows the answer to the questions immediately, and needs to probe the client a bit to find the problem, voice calls could be preferred. Ho

If the support staff knows the answer to the questions immediately, and needs to probe the client a bit to find the problem, voice calls could be preferred. However, if the support staff needs to look into the problem a bit, and already has required information from the client, I think email is far preferable. Being on the phone is likely to put the support person on the spot to make instant answers, else appearing to lack knowledge), whether or not correct.