Activation lock on an iPhone 5c

Good afternoon, I recently purchased a second hand iPhone 5c.

The iPhone has been restored and downloaded the latest iOS, unfortunately once opened it appears the phone had been lost and there is a number to ring.

I immediatly rang the number to return the device to the owner, but the man said the phone wasn't his and he has never lost a device.

So how can I unlock the phone? Please I need help ASAP as I need to use this phone.

Thank you guys.