I have a personal phone, that I pay the monthly bill. However each pay slip I get a 'contribution' to my phone bill which over the course of the year pays for ~70% of my total yearly phone bill.
My signature and business cards have that number on it. Works for me. Some people have company phones, but I refused that.
It depends on your role but generally what you have described is a recipe for disaster from the company side.
Phone numbers are like email addresses. Employees shouldn't be in a position to leave and still be contactable at the same address, that point of contact should go to the company.
It varies case by case of course but I'd never let someone in a customer facing role use their personal email or mobile to contact customers.