This is actually a traditional thing on Houston sports radio. It started years ago when Jeff Van Gundy was the coach of the Houston Rockets, and cell phones were new enough that radio shows would make a distinction between calling on a landline versus a cell phone.
"[Name] on a mobile" is an immediate indication that the call is a joke/impersonation, and the host plays along and the and audience knows right away. I guess outside of Houston it's not as well known and so some people think it's real.
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