I learned that the hard way this weekend. Definitely call before you dig anything--not just footers and trenches and post holes--anything. If your little kid is going to be out in the dirt with his Tonka trucks, call and have them locate the buried lines first. I'm serious.
Here in Colorado, the Public Utility Commission only requires the phone company to bury their lines 6-12 inches. That's gardening depth, that's kids-playing-in-the-dirt depth, that's a depth people (like me) might not even consider real digging. But, take my word, any digging is digging. Planting a tiny sapling is digging. And rototilling is digging too--my next door neighbor cut through his phone line with a tiller.
So there's my public service announcement for today. Call before you dig.