What are some options for getting a very inexpensive cell phone that I am only going to use to contact clients of my therapy practice?

Update: I am only going to contact them if I need to cancel or reschedule an appointment. So I won't be using it very much at all. I don't want to use my official, personal cell phone, where I would have to press *67 to hide my number, because most people are not inclined to answer calls from an unidentified number.
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