(Sip API) only works on G711 over Android 2.3+. 
Also the phones supplied by carriers may have the SIP stack ( blocked or cripple.

We need a third party open source sip stacks in order to make our own softphone over Basic4Android.

There are different open source sip stack libraries as well as projects are available on internet. You can download the source code of that projects.
Here is the List of some popular open source sip stack libraries which allows to voice call over internet.

1.Jain sip (I think the best option):

There are different open source projects which have used these libraries in their projects.

1. Jain sip: Not used in a "famous" app.
2. Sipdroid uses MjSip
3. Csipsimple uses PjSip
4. Imsdroid uses doubango.

Open source SIP stacks: Android SDK's default implementation (API > 9)
Advantages : Documentation available. Easy to understand.
Disadvantages : Not all devices are supported due carriers restriction. Works on WiFi only. Can't change codecs.

Third party: JainSIP
Advantages : Oracle (Sun) project ==> Active development. Looks easier than MjSIP (more documentation).
Disadvantages : None

Third party: MjSIP
Advantages : SipDroid is built on it. (source code available) (red5phone is another project) 
Disadvantages : Not fully compliant with RFC?. Lack of tutorials (Javadocs available though).Development almost dead

Third party: Doubango framework
Advantages : IMSDroid is built on it. (source code available)
Disadvantages : Generated apk file size tend to be "heavy" (>15MB as mentioned here)