Sipdroid is one of the best SIP/VoIP software available for Android mobiles. It adds native SIP/VoIP to Androids Dialer/Contacts. Make and receive calls over WIFI/3G/EDGE with perfect voice quality and lowest delay.
See SipDroid.org for license, changes, source code, FAQ.
For Optimal Standby/compatibility calling Skype register with pbxes.org and your SIP accounts from there. Basic accounts are free.
You can easily make sip calls to be your dialer default method of calling. The other way you can use it, is by calling + after the number you are calling to make a sip call. The integration of SipDroid with the Android mobile dialer is priceless. The voice quality is great as well.
If you are looking for a SIP/VoIP application for your Android phone, you should start with SipDroid.