The other day one of my customers started having problems with their phone system, which is handled partly by me (the phone network) and partly by another company (the PBX and Voicemail systems). The PBX guys determined that there was nothing wrong with their equipment so it must be a problem with the network.
I went out to the customer's site and determined that it was not my network that was causing the problem, but the DSL line they use was to blame. So I contacted the DSL provider and discovered that their account was suspended. They didn't pay the bill! And the DSL line was for the accounting department.