My therapist eventually had a business cell phone. I only know that because if I called him and left a message at the office, he would often return the call on that phone. It was obvious is was a business-dedicated cell phone because it was essentially the same number as the office number but with a different prefix (or maybe last number; I can't remember). I never called that number directly; if I needed him, I called the office number and left a message or spoke the answering service who contacted him.