How do I do that.

Thanks.,

Type *#7370# during standby to do a software format
Type *#7780# during standby to restore phone to factory settings

If you want to do a full format

6600/6620/7610), the sequence is slightly different:
power the phone off,
press and keep down the keys green/talk + ’3’ + ’*’, - power on the phone.
release the keys when the formatting starts..

I'll be doing format today .. But how do I backup SMS and Addressbook . Also I'ld like to know procedure of restoring them after format.

Thanks