Build an app

I am looking to identify a service provider who can assist me to develop an app, being a service provider with skills in in the area of building apps that work with your phone (ie apps like Smart Manager and similar that can 'interract' with your phone). The services are required to build an app that is intended to compliment the ordinary functions of your phone, so knowledge of how mobile phone functions work would be an advantage (ie how do the call, sms, email etc functions work on the phone).