I am developing an application for Android. For this task I use an SDK and Adobe Flash Builder. I am not an experienced developer so I had to look for help on the Internet. There found a page with relevant information aobut this topic. The page (if you are interested: http://www.voip-sip-sdk.com/p_456-voip-android-integration-voip.html)
also provided code snippets that handle the physical buttons and the rotation features of the mobile devices. I belive that the information provided by the page will also be usful for other developers.