Like the others have said, it's free, and you have to give them three days to come out and do their thing. You will find that some will come out and just leave a flag saying that they don't have anything in the area. Others will come out and leave flags and paint where they think their lines are.
This is very important!!! Take a picture of their markings before you do anything. While most are pretty good at their markings, they are not exact. Some are just plain wrong. If you have pictures, then nobody can come back and say you moved the flags, or dug an area that wasn't marked.
Be sure to know the rules on how close they have to be. I have to check every time that I have something marked, but I think it's three feet. If they flag a line, it's that and three feet of both sides of that line. Ask to be sure!!!!!!
If you have to dig anywhere near something, be sure to have a probe and a shovel handy. It's impossible to know how close you are to anything and just about as bad to know if you hit something. If you know you've hit it, then it's probably too late.
Good luck,
Eddie