The Find Duplicates feature is very useful to keep the phone book clean.
In my experience synchronization software duplicates or incorrectly overwrites information.
vCardOrganizer is able to find replicated telephone numbers (or any other data) on different vCards and lets the user decide what to do.
You can double click on the vCards pair to compare and fix them.
The Add Phone Prefix tools let you to add or remove a prefix string to all the phone numbers.
This utility adds Formatted Name property to all vCards.