How to deal with home customers in consulting business.

Hello,

I have a consulting business in the NY metropolitan area and was wondering how to deal with clients who after having me on-site call and ask for some phone support (for free of course) to help them with “some more issues”. Is it wrong to just tell them that I don’t offer phone support or “free” support for that matter? I don’t want to lose my customers but at the same time I don’t want to waste MY “free” time giving free phone support to customers.