There's really not set depth you can dig without calling the number 811 to have utility services marked.
Utility lines are usually buried 1 foot to 2 foot or more below ground level.
But it's always best to just call 811 at least a few days before you plan to do any digging just to avoid making contact with any utility lines.
The 811 call to mark utility lines is free so there really is no reason to not call 811 before you dig because it's the right and safe thing to do.
I always call 811 or call before you dig at least 3 days before I plan to dig anywhere in my yard or anywhere near the street or alley.
The call and marking of the lines is free so I would recommend just calling them to have them mark the utilities as you'd know where everything is for future reference as well.