When we make or receive a phone call in our building using a iphone 4 or 5 with AT

Hi; we recently installed a newer paging system in a conventional steel framed commercial building located in the small town of Franklin, GA. The problem we are experiencing is that each time a person uses an Apple iPhone 4 or 5 product (AT&T as the service the service provider) to make or receive a phone call it causes an electronic buzz or hum in our overhead building paging system. The building is approximately 150 feet by 250 feet with 30 foot high ceilings. The caller can be most anywhere in the facility and as soon as a call is made or received we get this problem. The sound is heard over all the paging system speakers and horns. We cannot reproduce the problem thus far with any Android phones we have tried and we have tried several. We would like to know if anyone knows a possible cause for the problem and or how to prevent. The paging system uses a new Bogen amplifier with 6 paging horns and 2 paging speakers attached. We have had the company that installed the paging system look at the problem and they cannot as yet determine a way to prevent the issue. Thank You.