PAX USED CELL PHONE INFLT.

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FLT WAS DELAYED IN CHICAGO BY 3 HRS BECAUSE OF SNOWY CONDITIONS AND MANY PAX WERE UPSET AT THE INCONVENIENCE. DURING THE DINNER SVC PHASE OF THE FLT; I WAS RETRIEVING AN ITEM FROM THE AFT GALLEY AND NOTICED PAX USING HIS CELL PHONE. IT WAS ILLUMINATED (ON) AND HE WAS PUNCHING IN NUMBERS AS IF MAKING A CALL. I POINTED TO THE PHONE AND SAID 'NO; NO; NO. YOU CAN'T USE THAT PHONE ON THIS PLANE.' PAX BEGAN YELLING AT ME AND STATED HE WAS ONLY PROGRAMMING NUMBERS INTO THE PHONE. I SAID IT DIDN'T MATTER; PLEASE TURN IT OFF. HE BECAME VERY ANGRY AND SAID HE WOULDN'T TURN IT OFF BECAUSE THERE WAS NO HARM IN IT. I PHONED THE CAPT AND HE SAID TURN IT OFF. RELAYING THIS TO PAX; HE THEN TURNED IT OFF; STILL ANGRY; STILL DEBATING. THEN A PAX BEHIND HIM SAID HE COULD USE IT JUST TO PROGRAM NUMBERS AND I PHONED CAPT AGAIN RELAYING THIS INFO. HE SAID TO ALLOW PAX TO USE HIS PHONE; BUT IF PROBS AROSE; WE WOULD COME TO HIM FIRST. PAX CONSIDERED THAT A 'THREAT' AND BECAME ANGRY ONCE AGAIN. HE STOPPED USING PHONE ON HIS OWN AFTER I SHOWED HIM OUR PA CARD WHICH STATES 'CELL PHONES MUST BE TURNED OFF INFLT.' AFTER INCIDENT WAS OVER; I ASKED FOR PAX'S NAME FOR MY RPT AND ALSO GOT 2 WITNESSES' INFO (NAME; PHONE; ADDRESS). SOME PEOPLE HAVE TOLD ME THAT A CELL PHONE CAN STILL PICK UP RANDOM SIGNALS WHEN TURNED ON FOR THE PURPOSE OF PROGRAMMING NUMBERS DURING A FLT. THIS NEEDS TO BE CLARIFIED. EVEN IF NOT; IT APPEARS AS IF A CALL IS BEING MADE WHEN THIS IS BEING DONE; AND BECAUSE OF THE SEVERITY OF ACFT SYS INTERFERENCE; IT IS RATHER UNNERVING FOR A CREW MEMBER TO COME ACROSS.