All it takes is a little reciprocation to help them get the point. Make a few calls to THEIR company-issued phone at random times - especially when they are on vacation and in the middle of the night when you get up after a noise.
If the problem persists, draw up a "fair use" policy and have the company CEO sign it and make it part of company policy. As long as it applies to all cell phones, everyone has the same rules.
PS - if you have a boss that insists that you are still on-the-clock after leaving work, tell them to their face that you need time away from work when you leave. If they can't respect that, you need a new boss.
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