Nickel 'n Diming
"Every time I call them, they bill me for fifteen minutes. I've stopped calling them because I know I'll get an invoice, whether they know the answer to my question or not."
People don't like being "nickeled and dimed", i.e. seeing an invoice filled with little charges. They find it annoying and it leaves them with the feeling that they didn't receive value for their money. It can lead to their going through and questioning every line of every invoice, as well as a loss of trust in the relationship.
Here's an alternative: don't charge for telephone calls. You want your clients to call. In fact, I make a point of telling clients that I don't charge for telephone calls or emails if I can answer a question quickly. If their question is going to take some research time or I need to come on-site, then I'll tell them. That way the client feels in control. I also advise clients to keep a questions folder or error log for issues that are too minor for a special call. If I'm on-site for something else, I can address the smaller issues while I'm there. The result is that I get more focused time with the client and they see value in the consulting charges.
Here's another handy tip: if you can do the work in the client's office without incurring significant charges then make the trip. Even if you have to go out of your way, make an effort to put in at least the occasional appearance and make sure you get around and meet the people you know. You would be amazed the number of times you will here, "I'm glad you're hear. I've been meaning to call you . . . "
You won't miss those nickels and dimes when you can charge whole dollars!