I'm curious about this too.
When I'm ordering online, I'm the kind of person that prefers to do it purely online, rather than by phone. But, I know that there are people out there, and probably a lot of them, who like to phone in an order.
I currently don't have a phone number on my site, but I've been thinking about it for a really long time. I have actually had customers email me and ask for a phone number so they can call in their order. I've emailed them my number, or called them, and they've really liked that. So, I've seriously considered putting a number on my site. The only thing is, I would never be able to answer the phone - the customer would get my voice mail, and I would have to return their call. So I don't know if it would be a help or a hindreance.
I've read that it can increase sales...but I'm still undecided.