Is there a VoIP / WifiCalling app that allows you to use your own number?
Many SIP or other online providers can route call outbound sending your Caller ID, or any Caller ID you chose for that matter (there are some legalities to this, so it could depend on your country or locale), but to get inbound calls over that service the provider must "own" the number in order to route it. The only other option would be to get a different number from that provider, and call forward always your existing number to that number.

A common way to do this is to port your number to Google Voice, and get a replacement number from your carrier... This allows Google Voice to route inbound and outbound calls on your number to cell phone. Google Voice allows inbound calls to be routed to your new number from your carrier transparently so the caller doesn't know the call is a data call or a voice call. This also removed the "link" between your number and the device, allowing you to make and receive calls via an Android app, iPhone app, Hangouts extension on Chrome/web browser, or with services like Simon Telephonics Google Voice SIP Gateway.