I use voip and voip only, no real cell service so maybe I can help. First option is an app called sipdroid in the market. If you have a google voice account and already have it setup with the official google voice app in android, then the first time you open sipdroid it will have a button at the bottom saying something like "link pbxes account with google voice" if you do that, it will make you an account @ pbxes.org, link google voice to it and save it in sipdroid. You can then make totally free voip calls and receive calls with your google voice account. Doing that in sipdroid is the only way I think there is to get a gv linked to pbxes for calls with a free pbxes account. But there's a 2000 minute per month allowancwe and pbxes enforces that pretty strictly, I have got a few accounts disabled and some completely removed for going over. Another option is grooveip(another market app) its not free, like 3.99 I think, it does the same thing as I said above and uses your google voice number for calls but I don't think there is a limit to usage like the pbxes option I first mentioned. And its a one time fee for the app, not a monthly fee. If you go this route, you only need the grooveip app and a gv account, no other apps or voip accounts are needed. Both options I mentioned above have worked great for me. Need anymore help or advice, just let me know!
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