Cable installations are like @#$%bleep, everyone has their own special way of installing them. Chances are the tech. who installed it did not document it, which could crucial for those "special" "fly by night" cable vendor configurations.
If there is a phone switch involved, you probably want to forget about it. If they are analog/POTS lines, a typical RBOC would use 66blocks, and have it labeled, yeah right.
Just try to figure out what you are getting into before you commit. For example, talk to customer, ask what type of phone service and or switch they have, and try to perform a visual trace. If it is well labeled and seems to make sense, cool, if not, I would be a little weary. For the weary situations, try to schedule a site visit around the same time the phone vendor comes in, you can learn alot just by watching.