I understand his reason. Changing a business phone is always "iffy," but it can be done if there is a lot of good preparation.

You bring it up the same way you brought up things in the past couple of days, avoiding accusations and being upfront with why you're concerned. I’d do it in person. You might even combine this problem with the still lacking NC letter. Thinking out loud here, how about proposing you and he compose a short TM reading something like, "Leave us alone," and you press the enter key to send it...or something like that?

Remember POJA and your communicative skills! Get his input on how to solve the problem before you make your proposal.