I know a guy who runs a business (not web related) who sends a bill (minimum of $20) just for picking up the phone, or sending a letter. Clients just seem to accept it, and he is very successful, but I still feel awkward about such things.
It depends. Normally I like to give advise and not charge for it, but when somebody expects text-book answers and responses, nearly every other day, and believes it's part of the support, then I suppose you would feel less awkward.
I can probably create a premium number charging .50c per minute, so I will still get paid without having to feel awkward. However, in terms of email answers, it all depends on the client. I had one client who asked me about search engine optimization advice, and basically suggested I teach them search engine optimization their site for free, as part of the support, which was not paid for, simply because they I did their site for them.
I understand we all have to work, and have to make a living, but why buy the milk when you can have the cow for free. Understandably mega-companies like Google Inc, Microsoft & Facebook can afford give such support, as they are making millions from other avenues which the masses are not aware of "If you're not paying for it, then you are the product"
The is a very interesting topic. Companies believe that they are entitled to this kind of support from internet businesses. The truth of the matter is that I cannot afford to spend 60% of my time giving advice unless it's billed for. Same might be said for chasing up clients (i.e. travel expenses). I recently learn a very important lesson, which changed the way I do business for ever.