This was driving me crazy!

ATTN GOOGLE APPS CUSTOMERS: You need to manually add a temporary gmail account to your android phone (This would be under settings -> “Accounts & sync”, “Add account” -> “Google” -> “Next” -> “Create” -> etc…). Once that happens, you will see the button on the home screen of sipdroid to configure your PBXes account.

Use your temporary gmail account username and password. You will ALSO need to select a username for PBXes.org.

Once this process completes, you will now be able to use outbound calling ONLY since your temp account will NOT have a Google voice number, but who cares, keep reading…

Log into PBXes.org with your username you selected on registration an the password from your temp gmail account, re-read that twice.

Once you are in, select “trunks” from the left menu and click on “GTALK/tempgamail@gmail.com. From there, change your username and password from the temp gmail account to your Google apps account. Click “Submit Changes”.

From there you may have to reload Sipdroid, but outgoing and incoming calls should work fine. Be certain that Google Chat (your google apps mail address) is selected/enabled as a forwarded phone (inside Google Voice web portal) in order for inbound calling to work on Sipdroid.

Feel free to delete your temporary gmail account from Google’s site AND remove the account from your android phone as a sync account. This would be under settings -> “Accounts & sync”, select the temp gmail account, and touch “Remove account”. Cheers.

Info from here:
http://androidforums.com/android-applications/252222-free-voip-calling-using-google-voice-sipdroid.html#post2152875 Thanks grimlock
and
http://code.google.com/p/sipdroid/issues/detail?id=753#c8
and
http://androidforums.com/android-applications/252222-free-voip-calling-using-google-voice-sipdroid-4.html#post2447641