I don't look at it that way. I always offer the usual free advise over the phone before I come out. If it's a slow drying complaint, I usually tell them that it's the exhaust vent and to have it cleaned out since I don't do that kind of work. 99% of the time, that resolves the issue. What's really cool is that they call me back for some other problem to service or they tell their friends that I'm 100% trustworthy enough to call. I'm getting more calls from customers who found me through their friends experience.
Don't worry about the bottom line too much. Work on being helpful over the phone first. Understand the problem over the phone and if it's something that definitely needs a service call, then go for it. Remember to charge accordingly. Your skill set is very valuable and you should be compensated properly. Don't under charge or you'll go broke.