Do you want to change the prefix of numbers in your contact list? See example:
Number: 0980000030
Old Prefix: 0 New Prefix: +84
=> New number: +84980000030

Number: +84980000030
Old Prefix: +84 New Prefix: 0
=> New number: 0980000030

Number: 0980000030
Old Prefix: (Empty) New Prefix: +84
=> New number: +840980000030