I think your best best is to just have a good voice mail system on your cell phone and then check for messages when you can (lunch, breaks, etc.). As long as you're returning calls within a few hours most people won't get upset at you.

Another option is to be accessible to email so you can answer them quickly - if you don't have access to email all the time at your day job, then you might want to consider getting a PDA with email capability so you always have access to your emails. You can post your sales and support email addresses on your Contact Us web page or Support web page and list your company phone number on your About Us web page.