Not a cell phone faux pas (we never turn ours on except to make a call, so nobody would be able to reach us that way anyhow), but in my last few years of working I referred to another manager, and to a policy he had put in place, quite disparagingly while on the phone with another employee. Turned out she was on speakerphone with the culprit sitting beside her in her office. Perhaps he took the hint, though, because he never referred to the incident and rescinded the policy shortly thereafter.