iPad 3G only allows internet connection on SIM but it don’t support call and SMS text message, but this problem now is solved by iPhoneIslam Team, as they hacked iPad 3G and enabled on it call, SMS text message and Facetime on iPad 3G with PhoneitiPad cydia tweak, first you will need to jailbreak your iPad to be able to download PhoneitiPad from Cydia, also the Cydia tweak works on iPad 2 3G model but you will have to wait some time as the Jailbreak still in Progress, iPhoneIslam will release the tweak in Cydia soon.
You can watch this Video which shows you what the tweak is able to do :
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