Both are correct.
However in first case "хотя бы" is applied to "мог", while in second case "хотя бы" is applied to "позвонить".
So... first one is better suitable for expressing idea, that he could make the call. It's more like satisfaction or wonderment about the fact that he has ability to do this call. Something failed to prevent him from doing this. Note, that latter doesn't implies that he really did it. Just fact of ability.
Second one is better suitable for expressing idea, that the call was the thing which he could make, but he didn't. It's more like regret or anger about fact that he didn't make call, despite of his ability to do it.

However, these things can be changed by stressings in living speech. So, context sometimes can be more important.