Is there a VoIP / WifiCalling app that allows you to use your own number? Is there a VoIP/Wifi-Calling app that allows you to use your own phone number?  (that is, the cell phone number that is already assigned to your device by your carrier)?  

I've been poking around online and it seems almost all force you to take on a separate phone number for the VoIP/Wifi-Calling app.  I've only seen one claim that Vonage allows it, but none other.  Ideally I'd like more than one to choose from.  Haven't tried Vonage as of this writing but going to try it right after posting this question.

I tried Vonage.  It does use my number for outgoing calls, but incoming calls still ring the phone app installed on my phone by Sprint.  I have a feeling that acejavelin is correct that my only option is call forwarding to another line (like google voice) which is inherently set up to work over IP.

Seems to me, ideally, if I could just actually replace the phone app that my phone came with, that would work, if I could find a phone app that can do that and that does everything I want.   By the way, the main reason I want this is to be able to use Wifi calling when I'm in a location with a weak signal but available Wifi.  Sprint offers Wifi calling but they have some bugs;  I've escalated to Sprint's Engineering team, but it does not appear they are going to fix their bugs anytime soon.
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.