I work outside for a major telecom, doing every type of phone line repair as well as marking said lines. In my state, Pa, the utility company mark is good for 18" on either side of the painted or flagged mark. Meaning if you hit the line within 18" of the painted or flagged mark, you get a bill for repair. If the marks are way off, and the line is damaged beyond 18" away from the mark, the phone company repairs no charge. Now remember, the buried phone service line, the one going to your home, is very thin, like the thickness of your pinky finger thin. A spade will slice it in two and you won't notice till you try to make a phone call. The spec depth for a phone line is 18" deep. But I have seen some only buried 3". Take your time around the marks. And pick at the trench with the shovel turned backwards as you get close the the marks. Even use a screwdriver to find it. Once you find it, take it easy around it and take care back filling around the phone line, no rocks or big clumps of dirt that could smash, stretch or break it. Also you can always call the utility and have them send someone out to check and remark the lines as you get close to the marks. Also newer lines can be in a conduit or maybe direct buried like most older lines. Good luck! Oh ya, If you have fiber optic phone service, be very, very careful with those. If you cut one, they can and will blind you if you look into it, looking for the "light", which invisible and you can't see it anyway.