Question: Since many days I am trying to achieve this, but I can only delete/ clear all recent dial, received phone numbers.

Is there an option/ hack to delete only one number at a time on my iPhone ?

I have iPhone 3GS

Answer: 
Go to 'recents' on phone. Tap 'edit' in top right hand corner of screen.
You will then have like no entry sign type symbol at the left of each number in recents. Tap the no entry sign by the number you wish to delete, a box will appear to the right of that number asking you if you wish to delete it, then type delete.
I doubt iOS allows you to enumerate recent calls through standard iOS apps. I would suspect that through jail-breaking you could make an app yourself that could do it. Other than that, I doubt it's possible at all.