Beeps during conference call

My iPhone emits a loud beep every 15 seconds to everyone else but me when I'm the creator of a conference call. The beep continues for the remaining person on the line when everyone else has left the call. The beeps start, and persist, if the call was at any point a conference call.

I've tried moving my sim card to an android phone - this silences the conference call beeps completely. I interpret this to mean that the beep is in fact produced by the phone itself. My carrier also denies any knowledge of the problem.

I'm running iOS 9.3 and have seen this problem persist through several updates. I would very much appreciate any help with this as it doesn't seem to be a problem that many people share.