From Chris Gummer:
Reverse Contacts is an application designed to make your iPhone or iPod touch's contacts searchable by phone number. Reverse Contacts gives you a reverse address book for your device's contacts.Maybe you missed a call on your land line at home or work? Maybe you've got a phone number on your phone bill that you don't recognise? If so then Reverse Contacts can help you! You can enter a full or partial phone number into the application. If any of the contacts on your iPhone or iPod touch match, the application will list the matching contacts.Once you've found the matching contact, you can edit or use the contact's details. For example, you can display the contacts full details and dial their number (iPhone only) or even send them an email.Reverse Contacts has the following features: Searches your device's contact by phone number Immediately lists matching contacts for each digit entered Full or partial search capabilities Displays the matching contacts' names and phone numbers Display matched contact count Matched contacts can be edited Matched contacts are displayed according to your devices configured sort and display order Application starts with your previous searchImportant Note: Reverse Contacts can only search contacts that are contained in your device's Contacts application.