I would suggest looking at it from the perspective on what type of phone support is required in general depending on complexity of application and real-time access to information. For example, if the service is a SaaS based contact-centre solution, then you may need phone support for customer confidence/loyalty if they are worried about access or loss of data in real-time. If the service is more of an office productivity tool, then as the previous experts listed you need to look at the alternatives prior to someone having to pick up the phone (i.e. FAQs, manuals, discussion boards, on-line forms for various types of requests/feedback or even a LiveChat window).