The availability of 24/7 phone support is a cornerstone for companies that care about customer comfort.
You might get immediate interaction, but that does not guarantee faster resolution. I was with a provider years ago where their phone support consisted of level one techs who couldn't seem to answer the simplest of queries.
Let's clarify something. For the most part it depends whether people are going to be using your phone line for actual support or general sales questions. The very existence of this line does provide comfort, but if properly educated, people will gravitate towards a ticket system, especially if you address issues quickly. Unless a website is down and they are acting in panic mode, a line will likely be mostly used by new prospective clients calling in with a question or two, potentially to test the water prior to making their decision. We are big proponents to having a phone line, even if it has limited weekly hours.