While I don't believe that using a private cell violates any fairness rules, I personally wouldn't want my number on their caller ID.
I used to have an unlisted number just because of this. And still a couple of times a month, years ago when I was collecting, my dad who lives nearby would call and give me messages of folks who called him, looking for me. Yes, with my last name if you find another person with it, we have to be related.
My opinions are not necessarily my employers.
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell