Crypted Android SIP - App

We would like to have a custom SIP app developed for the Android platform, It will contain SIP VOIP features in the background to make calls to our VOIP servers. Front-end will need to be a Dialer and if possible to integrate in the native android dialer (like CSIPMPLE app does). This app ofcourse needs to be native, no web stuff. We would like to have this Android app for iOS as well, but priority is on the Android version.