I'd go with Hal's suggestion. The most practical way is to wait until the problem reappears, then disconnect at the network interface and connect a phone direct to the incoming line (either your butt set or get the owner to plug in a regular phone if he's doing it when you're not there). If you still get noise, then the problem is on the TelCo side.
Here in Britain we can dial a test number which runs an automatic routiner on the line, looking for basic faults such as low-resistance to ground etc. It doesn't always identify every fault, but in a large number of cases of noisy lines you can bet that the tester will come back with a result which indicates it's TelCo plant problems.
