NETWORK MODEL-- K 750 (SONY ERICSON) NO ACCESS TO NETWORK (PROBLEM)

Dear Sir, I am using our Airtel sim Hand set Sony Ericsson K750i past 10 months.now i am facing one new problem to me its display's NO ACCESS TO NETWORK. SO Kindly guide to for problem soultion my hand set was underwarranty i replaced new main PCB also but problem could not solve. so kindly support me to solution.my cell no:-9789658993 Note : - now call to this no:- 9994010114 or reply this e-mail id
thanking you,

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