From reading your replies to other comments, it sounds like you want access to a phone's local resources, and you are possibly interested in using web technologies. I would look into Apache Cordova (aka PhoneGap) or something similar (there are many). This allows you to build once with HTML/CSS/JS, and then distribute via Android/iOS and more (differentiation being each platform would need specific front end JS code to access the phone's local resources). This approach is commonly used in conjunction with a back end that accepts API Requests (aka front end makes Ajax request, back end responds with Data). I'm using this approach for one of my current projects.
I would suggest to only go for the platform where you're most comfortable developing with. Web app can be good indeed, but if you already know objective c or java then you'll be probably better off going for native iOS or Android first. Good luck!
You might find parse.com services cover your user and notifications requirements, and perhaps even your REST backend.
http://cordova.apache.org/docs/en/2.5.0/cordova_camera_camer...
And maybe the dialer: