No hacking through WhosHere
As the creator of WhosHere I can tell you that this rumor is 100% false.
All iPhone applications run in a protected mode that prevents them from accessing most information on your device (including you cell number).
But beyond that WhosHere was written with anonymity in mind, we don't even collect email addresses of our users nor do our servers even know your precise location (obscuring occurs on the device itself).
In fact WhosHere was designed to be as far from a phishing app as possible. It's something we took a lot of care to ensure doing the development process.