The network transfer protocol of UDP or TCP has nothing to do about it. The noise is actually a hardware noise which some people diagnosed to the iphone lens focusing repeatedly.
It happens after the call has been completed, the zoiper application does nothing, sits in background and maybe waits to receive a call if there is one on the VOIP channel.
I am a developer and I know network protocols, please investigate in detail this bug in the application, a TCP or UDP protocols have nothing to do with the phone making a noise, which may be either a speaker function left over somewhere or an incorrect request to re-focus the camera.