Here are the steps for getting free calls over wifi or 3g on your android phone.
First: goto http://www.ipcomms.net/ and signup for a free account. it usually takes less than 10 minutes for them to activate the service.
Second: Signup for or activate a google voice account and install the google voice app on your phone.
Third: Download Sipdroid from the Android market on your phone. Now hopefully they activated your service from ipcomms and sent you an email with the information needed to set up sipdroid. If not then you have to wait.
There are four items of importance in the email they sent you, these are: The phone number, Username, Password and domain/proxy. These must be entered into sipdroids settings. (settings/sip account)
Fourth and Final: If everything is all set now all you need to do is login to google voice settings and add the number that you received from ipcomms to your forwarding list. Tip, if you have $1.99 to spare on Google Voice dialer it makes it possible to dial right from your phone instead of the website but it is optional.
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