How important is a Phone number (1-800 or regular) and a mailing address? I would say that the phone number can add credibility, but I am not sure if anyone would use a mailing address.
I assume a lot depends on the market you are targeting, if you are targeting businesses that are not very active on the web, then yes any additional information will help There are millions of businesses in Asia and around the world that are still using joint web services where they have a web page on a web site provider service site that says example :- ABC Company abc@abc(dot) com So yes these type of business might pick up eventually on how strong web presense can increase substantially their business, and decide to get more indexing for their products, hence they may make contact by other methods i.e. phone and address
Regarding our type of business i dont think too many people would be looking for a mailing address, but i guess willingness to provide one if requested would be polite. Although missing from my own at the moment, i think a phone number does have its place if the owner would like to provide it, saying that, if someone is also determined enough they can get a phone number from whois.