Android Zello works with WiFi, but not with 3G.
You may not have a mobile data plan enabled or your settings may not be changing. Open Settings > Wireless and networks > Mobile networks. Make sure the box next to “Use packet data” is checked.
Also open “Access Point Names” and make sure that "Internet" APN is enabled. Choose "Internet APN” and verify that the configuration matches the APN required by your service provider.