Based upon a Java SIP stack contributed by MJSip, SIPDroid is currently in public beta.
From the SIPDroid.org website:
After completion of the closed alpha stage this project will publish the software for free under the terms of GNU General Public License v3. The first beta version will be for software testing. So please allow for some issues and incompatibilities at the beginning.
Although SIPDroid will likely mature quickly, it is currently only fully supported using virtual PBX service from PBXes.com. PBXes.com offers a free basic account registration for their service.
Once you have created a basic account with PBXes.com you can set up additional SIP providers/registrars within the Trunks section of their web based UI.
PBXes allows you to register several trunks from multiple telephony service providers of your choice. PBXes routes incoming calls over SIP and the PSTN to you. If you are online you can take a call as VoIP, and if you are offline the call will fall back to GSM.
Beyond their free basic service, a paid account additionally allows for handoff of calls between networks. PBXes also supports NAT.
To install Sipdroid you need version 1.5 “Cupcake” of Android. It is already available from HTC for Android Developer Phones. Visit this link for details on updating the OS. An OTA (over the air) update for the other phones has been announced for coming in the beginning of May.
Tragically, most of us here at The VoIP Insider are Apple iPhone users, but we have procured an Android mobile phone from Yannick Tessier, our head of engineering, for testing purposes. We will attempt to get SIPDroid working this week with Asterisk and let you know how we fare.