37signals blog has a great post on phone vs email support.

Yes. It is totally possible to provide great support without a phone number.

Jitbit Software gets a number of requests every week from people who would like to use phone instead of email. But emails can actually provide faster support then a phone call. A phone call requires you to stop from what you're doing, find a quiet place and listen to hold music for an hour. And after all in 90% cases your phone call will be simply logged by support team as an issue in some web based helpdesk system.