WriteThat.Name aims to keep Gmail users from ever missing another change in their contacts' phone numbers and addresses. The tool uses semantic technology (a.k.a. robot readers, not human readers) to monitor any changes made to the signatures of incoming emails and updates address books accordingly.
If you're skeptical about handing over that much control, you can opt for the manual setting which allows users to select which details must be updated in their address book. Further, an optional Notification setting can be turned on so that the user receives an email alert anytime changes are made.
Note that no data is actually deleted. If one of your contacts changes the contact number in their signature, Kwaga simply adds it to the previous number recorded. You can then go in and manually delete the old one, if it is in fact defunct.
WriteThat.Name requires a Gmail or GoogleApps account. Interested parties can get the feature for US $3 per month or US$20 per year for each subscribed email address. Check it out here.