The current Zoiper for IOS seems has a "bug". Once the app hangup a call (end a call) and push it to the backgroud, few miniutes later, you will hear the noise from iphone speaker. If I activate the app again, means bring it to foreground, or kill the app, noise dissapear!
It's very annoying. Hope next version can solve the problem. Tested on iphone 6, IOS 8, with many different devices.