I am not aristocratic, nor hob nob with such a coterie, to provide you with 'the most sophisticated' response; but here goes:
"Oh, hello, matey. Bad timing, I fear. Unlike good red wines, I do 'travel' - like, right now. So, dear boy, can't parlez vous at the moment. Can we catch up later at HRH's over cocktails? Hate to be a drag but the gendarmerie, you know."
However, fortunately for me, I think you mean 'the most colloquial yet 'proper'' response.
If the person is a native speaker and a friend:
"Oh, hello Paul. Look - I'm on the road at the moment. Can you call me back in, say, half an hour?"
Oh, hello Paul. Look - I'm in the car at the moment. Can you call me back in, say, half an hour?"
"Oh, hello Mr. Smith. I'm sorry, but I'm in the car at the moment. Are you able to call back in, say, half an hour?"
Neither mention 'driving', but the native speaker understands that this is implied.
If the person phoning you is not a native speaker, and not really fluent, then:
Oh, hello Paul. Look - I'm driving at the moment. Can you call me back in, say, half an hour?"
Other posters will give you a few more choices.
Note: I am making a polite request...so why don't I use 'could'?
This isn't just a polite request - for the caller's part, he might have to go to a business meeting and be tied up for hours. You actually do need to ask if he is able/available to ring back later.
Note: there is a subtlety in meaning between saying, 'Can/could you call me back..' and " 'Can/could you call back..'
The first adds a personal note to the conversation. The second, omitting the 'me', is what a receptionist would say when dealing with a call that another person is unable to take - less 'personal'. So, if you said, 'can you call back' when speaking to a friend, it sounds 'stand-offish', a little 'distant'.