Daniel,
What I and Saúl understood after thoroughly going through the Pull request and other comments is that K9 team is looking for some kind of plugin architecture. They don't want to implement SIP as a hardcore functionality for every user.
We are trying to make an interface where a user can install an additional application such as 'K9-SIP' and when this application is installed dial with SIP would be added to user menu and it will call K9-SIP activity to place a call.
This way other developers would be able to develop alternative apps for K9 SIP on pre-defined guidelines.