I'd ask about resource/memory management, because it's an important subject in C++, and it doesn't require concrete code. Just sketch a simple hypothetical scenario, and ask how they'd ensure some vital resource gets freed even in the face of errors/exceptions. Say they're developing a network app, how do they ensure that we close our sockets properly? Of course the proper answer would be to wrap it in a RAII object, but don't ask them that directly (it's easy to google "RAII", while the above question "how would you ensure resources get released properly" actually shows you whether or not they know the appropriate techniques. If they answer "wrap everything in try/catch", they might have a problem. And this ties in nicely with questions about the differences between heap and stack.
You might be able to come up with some simple question about exception safety too, which doesn't require any real code. In general, I'd say a discussion of all the various C++ idioms might be a good idea, because many of them don't require much actual code, but are still vital language-specific concepts.
See if they know about smart pointers (again preferably by giving them a situation where smart pointers are called for, and see how they would solve the problem), and maybe templates/metaprogrammin (in the latter case, probably just find out if they're aware that it's possible ,rather than requiring them to code actual metaprograms on the phone)
You might also want to ask about some common areas of undefined behavior (what are the values of a and b after executing a = b++ + b++??), or allocate an array of 10 elements, and add 10 or 11 to the array pointer, and ask what the result is in each case (+=10 is legal, gives you a past-the-end pointer, +=11 is undefined). Or give them a scenario where they need to copy a lot of objects, and ask how they'd do that (plain for-loop copying each element at a time, memcpy or std::copy are obvious answers. Note the caveats with memcpy, that it's not safe for non-POD objects)
Or ask about their coding style in general. How do they feel about iterators? Do they prefer plain old for-loops? Do they know how to use std::for_each or std::transform?
Edit:
Seems the a = b++ + b++ (the answer is undefined behavior, btw) suggestion in particular generated a lot of comments. Perhaps people read too much into it. As the OP said he preferred to ask concrete (not abstract, and easy to explain/answer/discuss over the phone) questions, that'd reveal a bit about the interviewee's C++ skills, and this is a simple (and yes, perhaps nitpicky) example of that. The reasoning behind it is that 1) it has an intuitive meaning, which is wrong, and 2) you have to have a certain level of experience with C++ before you realize this. And of course 3), it's short and easy to ask over the phone. It doesn't require anyone to write code down. No, it won't reveal whether the candidate is a "great programmer", but as I understood the question, that wasn't the goal either. If someone gets it wrong, it doesn't mean much at all, but if they get it right, you can be fairly sure that they know a bit of C++. But if you read my answer again, you'll see that it was just a quick example of a category of questions I thought should be represented. C++ is full of undefined behavior, even in code that looks completely harmless and intuitive. Asking the candidate to recognize some instance of this may be useful, whether it's the "modify the same variable twice in the same expression" example above, or something different.