First, not everybody can switch back and forth at the whim of just one of the partners. Not even switches.
I would think that you need to ask yourself if this is something you can and want to do. Remember that we can only give what we have in us to give.
Admittedly it is not always easy to find out, as to many people starting to dom means getting over a hurdle. But still, if you feel you cannot, or cannot neccesarily when he happens to wants to, or only can online, then I think you need to tell him.
How did it feel to do it online? Was that ok?
I'd say doing on the phone is a hard way to start.
If you want to do this, then I agree that you need to sit and talk about it, and let him explain in much greater detail what he is into - what he wants. Then you can see if you think it is something you can and want to do.
Do by all means present his answers here, I am sure we can give you some inspiration :-)
Finally, are you doing this because you are afraid of loosing him again? A powerful motive - but it doesn't work that way. If you want to, fine. If not, you cannot give it, but he will likely be ok with having you as sub.