Incoming call on 2nd iPhone without SIM Card
iOS 8 has this new feature called Continuity. Which basically means you can make and receive phone calls on your iOS devices as long as your iPhone is on the same Wi-Fi network.


Source: Apple

Go to the phone with the active cellular SIM and you can disable this in settings app:


Phone
Calls on Other Devices
Allow Calls (turn them all off or disable some but not all other devices if you prefer)