Your WHOIS details must be valid, but here are a few legal ideas to protect your privacy without using domain registrar privacy services-
Get a local PO Box from the post office, cost ~$5 per mo.
Get a free voice mail number from something like k7.net, or spend a little and get a nearly free "Skype-in" phone number from Skype.
Use a business name along with a title instead of your personal name. If you notice most large companies don't have a person's name in the WHOIS (it usually says something like "Domain Manager" along with the company name).