This app is the best FREE app for your contacts.
1. Help you to backup/restore/import/export your contacts to/from iCloud
2. CHECK and MERGE duplicated contacts
3. Add/remove the country code
4. Format phone numbers
5. Delete all contacts
6. Export contacts to Excel file
7. Share your contacts
8. Send SMS/Mail to a list of selected contacts.
9. Do you want to change the prefix of numbers in your contact list? See example:

Number: 0935675680
Old Prefix: 0, New Prefix: +84
=> New number: +84935675680

Number: +84935675680
Old Prefix: +84, New Prefix: 0
=> New number: 0935675680

Number: 0935675680
Old Prefix: (Empty), New Prefix: +84
=> New number: +840935675680