For android phone apps check out https://ringotel.co/ - they have multiple apps and integrate nicely. I'm even working on better integration myself for our internal processes.
If you are looking at making an Admin app for managing Fusionpbx and not just a phone app, that would be interesting and the official API might be nice for that.
I've had a pleasant experience with Acrobits too, they can whitelabel an Android, iOS, PC and MacOSX app for you and they are even willing to do all the publishing of the app.