(1) Displaying a phone number in your title tag might increase your click-through-rate for your search listing. That is if, the searcher is searching for your phone number.
(2) You can take traffic away from the Internet Yellow Pages that list your business's phone number.
(3) Optimize for other business's phone numbers and when they do vanity searches on their phone numbers, they will find you and maybe buy what you have to sell them.
The Sphinn thread lists other reasons why you might want to make sure you are well optimized for your phone numbers. For example, someone forgot your company name, but your phone number is listed on their caller ID. I tried to find companies this way in the past.
But if you place phone numbers in your URL, be careful not to end in a zero.
Forum discussion at Sphinn.