In my humble opinion, I think a better question to ask is - "How do I answer every call, and make sure I'm not wasting time with 'tire-kickers'".
The very first rule you should set for yourself is "Every call get's answered by a live voice". Notice I didn't say your voice. You are experiencing a good problem. It's good that the phone is ringing. Bad because you didn't anticipate the work and time lost sorting out the rif raf.
That said, I agree with the others. The 800 number has no use anymore. Originally it was intended to keep incoming calls from the customer free. In this day and age, I don't know a single person who chooses not to make a call because it's long distance. Especially with voip.
Yours is a problem of qualifying. That takes effort, and expertise. But there are ways you can solve this. First off I would consider hiring a VA to answer the phone. This isn't always the best answer, but there are times when it's appropriate. You want this person to be able to qualify the lead, perhaps they have a script, or an informal survey they give in order to know how to handle the call. What you want to know is that every call that comes to your cell phone is a qualified lead. Using a VA is a smart way of getting that done.
Or you could consider other ways of qualifying leads. Instead of making the call to action a phone call, why not send them to a landing page on your website. Off the top of my head I'm thinking the landing page could be a small questionnaire or a contact me webform that, once submitted, will connect both you and the user by phone.
That one gets a little more complicated, but it is very doable, and not nearly as expensive as you might think.
In summary. Yes every call absolutely needs to be answered. You just need to create a reliable system for qualifying these calls before they ring through.