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Exactly,
You can basically do it with just Goolge Voice and SIPgate...skipping all the pbxes and sipsorcery stuff. Using the those configs (pbxes/sorcery) lets you initiate the calls from sipdroid. However, if you just initiate the call from Google Voice instead (sipdroid will receive the call), then you are all set with just GV and a SIP account. You can either initiate the call from the GV mobile webpage and an Android app called GV Dialer. Both of those can be configured to initiate the call via your data connection - whereas the official Google app always makes a phone call to start the call.
Ohh - and I just stumbled across another GV dialer program a few days ago here