My family has been in the business for over 30 years and our ground rule for estimating is absolutely no estimates over the phone. We MUST look at it in order to price it because we have to know what we are dealing with, size is not the only thing that matters. That's why it is crucial to look at the job before the bid. And get a firm estimate from them, this shouldn't cost you anything. Go from there, and DON'T just pick the cheapest bid.
Cheap price means cheap quality.
Look at their reputation and possibly check out some of their work.