Contacts Mate is a small mate tool for the build in Contacts app of iPhone. As you know, the build in Contacts app can not search persons by phone numbers. Without this app, you can not know a given phone number is existing or not and who is the holder for this number.
After you get the search result, you can go into the detail page. In the detail page you can make phone call/send mails and edit the contact directly, every thing you do is just like in the build in Contacts app.
Just download it, I am sure, it will be very useful in sometime.