I think your on the right track Sam.
Another way that we have checked these type issues is to take a good true RMS voltmeter that has a PC link.
Hook a ole cheap laptop that you have laying around to it and let it run a data logger for a week or so and then compare notes.
We had one a few years back that had a "Wild Leg" on their 3-phase that was causing about the same problem.
I still think this is a power issue, instead of phone equip.
