Without deleting your full history. Dont think this needs too much explanation. lol.

Whats wrong with deleting history? It doesn't delete contacts just recent calls list.  Hopefully your not using your Iphone with a calling plan thats registered to you as your "HOBBY" phone.

You might try software that will archive what's on your iphone. You can hide this file for reference and then wipe your previous call history.

If you're on a Mac, try PhoneView: http://www.ecamm.com/mac/phoneview/

Windows, try TouchCopy: http://www.wideanglesoftware.com/touchcopy/index.php

Not 100% sure this will work, but it's worth downloading the free demo and trying.

Just be careful as to when the last time you synced with iTunes. It creates a backup of your phone, and if you have to a restore, it could bring the number back.

recent calls will purge after so many calls. Call a bunch of your friends and just leave a message for them to call you back. Voicemails are kept even though they are deleted, until you go into the "deleted messages" and delete them from there as well.

Since you're on AT&T, you may want to purchase the service that lets you block numbers. It's extremely handy for when you accidentally use the iPhone instead of your hobby phone.

No way to delete individual calls. But as was previously said this will not delete your contacts. Be very careful using an iPhone as a hobby phone. I would advise against it since they have a bad habit of retaining information. It will retain all email addresses in its cache even if the message is deleted from all folders and trash. The address will continue to pop up when entering addresses, even if contact no longer exists. The only way to clear cache is to reset phone to factory default and sync the device again.

Really, how many celebs have been caught by their phones?  
Get a hobby phone.  Keep your regular phone clean.  
Use a separate private email account (different P/W).
skb