1) Do you mean you want to implement a SIP app? Then, check this: http://developer.android.com/guide/topics/connectivity/sip.html
Make you app and listen to ACTION_CALL.
Or
2) Do you want to invoke a calling app? Then the usual ACTION_CALL will do. (For example, in my phone ACTION_CALL will prompt me if I want to use Skype or Phone App)
Added: I am using SipDroid in one of my phone. This is what happens when I try to make call: