"Do you have a cell phone?"
the answer is "は" on the question.
I've been wanting to know how to ask "do you have a microphone" online... and one time i said
"マイクを待っていますか？" and the person understood and answered,
but I've also heard a Japanese person ask another person
was just wondering what the most natural way to ask this question is (and if it sounds a little foreigner-ish to say 持っていますか)
No, because it's not talking about the location of the phone. Here arimasu means have. It's a perfectly valid way of asking someone if they have something and it's just as commonly used as motte imasu. In fact motte imasu is probably more accurately translated as do you have it on you right now/currently.
I dont know why you are getting so many downvotes. In uni, i learned "motsu" is "have" as in like holding something, and that "ga aru" can mean have, but usually means possesion.
Instead of downvoting alone, maybe leave a well constructed rebuttal alomg with where you get your info.