Ask HN: Replacing native call/text Android functionality

I'd like to replace the native call/text functionality in Android. I'd like to build an Android app that will handle incoming/outgoing calls/texts using Twilio. A wifi-only phone of sorts. I've built Android apps in the past so I'm familiar with the process. My knowledge of modifying code on a rooted android device is lacking. What I need help finding is how to override/hack the lock screen to show incoming calls. Incoming text messages can be handled with normal push notifications. Lock screen widgets were removed in Lollipop so I can't go that route. I'm looking for any pointers, helpful info, code examples, or comparable apps. I'd like to make sure this is feasible before committing any amount of significant time to it.