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A simple SIP implementation for Android using a 3rd party SIP stack (not the native Android SIP library, preferably PJSIP). The client needs to support Android API 9+ and function over both Wifi and mobile network data.
It is an already existing application which receives the SIP server and channel number from and API, what is needed is only connecting to the SIP server to make a predefined call; no dialer keypad is needed and the application doesn't need to receive calls.
The existing application is developed using native Java code and Android SDK so the added functionality needs to be the same.