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: 
You will have to wait for iOS 5 to be able to do what you are asking. I'm running iOS 5 and I'm able to do this without a jailbreak.
![iOS 5 Developer Preview](https://i.stack.imgur.com/NYCEh.png)
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.