mobile networks not able to register...please help

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http://androidforums.com – hi! all i have a chinese tablet with android 2.2 Model number: wondermedia WM8650 the problem is that when i insert sim card its showing mobile network signals but when trying to make/receive phone calls its nt doing so. when i m manually trying to search for mobile network it shows the mobile network....again when i try to register on that network it shows message " unable to register network at this time" . i have tried 2-3 SIM cards from differents mobile company...please HELP wat to do..:confused: emsi me: [email protected] (General)