If it is faulty FiOS equipment, you would not be charged. If it's inside-wiring, you may be charged (example: telephone wiring or new coax run).
You may be charged if it's an issue with your equipment...like lets say you cannot get a picture on your TV, you go through and troubleshoot with an agent but you refuse to check cabling. If a FiOS tech comes in and plugs in your TV, you are going to be charged for the visit.
I'm not covering some other special instances but it's really common sense if you think about it.
I hope that helps!