This was the quick way of get SIP integration for Appcelerator's Titanium. From Gingerbread onwards Android comes with a SIP stack/API. So I'm just tapping into it. It is not ideal. Firstly it doesn't support a separate Authentication UserId. I think that is from about API 13 onward... Also I'm not terrible confident with what Google has provided.

Here is how to  use it.
You need to include the module in the tiapp.xml:
            <module platform="android" version="0.1">com.yydigital.sipclient</module>

You also need to add these permissions:
            <!-- For Basic Sip -->
            <uses-permission android:name="android.permission.USE_SIP"/>
            <uses-permission android:name="android.permission.INTERNET"/>
            <uses-permission android:name="android.permission.VIBRATE"/>
            <!-- For StartAudio -->
            <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
            <uses-permission android:name="android.permission.WAKE_LOCK"/>
            <uses-permission android:name="android.permission.RECORD_AUDIO"/>
            <!-- For Speaker Mode -->
            <uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS"/>


Then the code is something like this:

        var sip = require("com.yydigital.sipclient");
       
        var sipClient = sip.createSipclient({
            username : ...,
            domain : ...,
            password : ...,
            outboundProxy:...,

            autoRegistration : ...,
            sendKeepAlive :...,
            port : ...,
            // Events
            onregistering : function() {
                //callback
            },
            onregisterationfailed : function() {
                //callback
            },
            onregistrationdone : function() {
                //callback
            },
            oncallestablished : function() {
                //callback
            },
            oncallended : function() {
                //callback
            },
            onincomingcall : function(e) {
                //callback
            },
            onringingback : function() {
                //callback
            },
            oncallbusy : function() {
                //callback
            },
            onerror : function(e) {
                //callback
            }
        });
       
The following methods are supported: register, answer, close, hangup, holdCall, initiateCall(address), sendDTMF, toggleMuted, unholdCall.
The following  get properties are support: registered, incall, muted, onhold.
The following  set properties are support: speakerMode.

This is what you will need to do on your own:
All audio files: ringback tones, ringtone, dtmf.
All UIs - meaning everything.
Make the phone wakeup, turn on and launch application on an incoming call. I needed to build another native module for that.