Do you offer phone support?
Yes, however we do not offer toll free phone support.
Do you think all hosts should offer phone support?
Personally, I think any company should offer phone support regardless of the service they offer.
When do you think they shouldnt?
When the "company" in question is not really a company, and is merely someone with too much free time on their hands.
Do you think customers would signup just on the basis that phone support is offered?
It depends. If I were a client, then I'd only sign up with a company that was a) a legitimate registered company, a) they showed the address to their office and c) offered phone support.
Personally, I do think that any company should offer phone support (not specifically toll free) no matter what service they offer. If you're a legitimate company, you technically should offer phone support to your clients. I do understand that in many cases, it's simply someone who has another job where they work 40 hours a week and therefore simply do web hosting as a hobby - Although I am against things such as that, I do understand fully that those people are unable to offer phone support to their clients due to many reasons; mainly them being at work all day and wanting to sleep at night.
One thing you need to ask yourself though is, if you owned a large registered company and were looking for a service from another company - wouldn't you feel uncomfortable going with a "company" that was simply someone who had a day job but enjoyed doing web hosting at night, and did not offer a company address nor offer telephone support?
I know I wouldn't be comfortable with that, and I'm sure you wouldn't be either if you were in the same situation.
Small note: There was a lot more here, however it timed out so I had to re-write the post, and therefore a lot is missing as I didn't re-write everything 100%.