Having use One Call numerous times in the past I can confirm that they will only locate lines that are owned by public utilities. For anything that was put in privately, you need to hire a private locator (which isn't cheap for a private citizen). Sometimes the private locator is even the same company (or person) who was locating the public utilities, but working under a separate contract.
Usually residential properties don't have a lot of private utilities, so in most cases you can get by without the private locate, but a line connecting a garage to the house probably would be private.
Of course sometimes One Call messes up too. Several years ago I was putting some test holes on a commercial property with a pretty major gas line. I had another locate done at the same property a couple of months later and it turned out I'd just missed the line because the original locator had said it was in a completely different place (apparently he was fired for incompetence, but that would have been small consolation if I'd hit the line).
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