The entire idea of "summoning" or "calling on someone's (especially a sentient's power" is frankly moronic beyond its physical consequences. I can buy Fried helping Caro since they've known each other for a long time, but why Voltaire will help Caro is frankly beyond me. Frankly, I don't even want to think about what happens if ten people call on Voltaire at once
However, from purely
physical point of view, it is not much more troublesome than transporting. In fact, Caro implies that the two are highly related skills anyway. Summoning is most likely just a form of enforced teleportation. It probably goes something like this:
1) You chat up your summoned creature and make a contract with him.
2) When you need him, you use the spell to activate the link, find out where he is, and grab him by teleport.
So that leaves what is Fried. I think the little Fried we see is kind of a natural device, a little like Reinforce II really but made by Friedrich (remember Rein was supposedly made with a small piece of Hayate's Linker Core). Friedrich's real body is stuffed in the world of Voltaire and Co (speculation here but methinks the summoned creatures' original world is the one where Caro, Erio and Lutecia lived in at the end of StrikerS). When it is needed, Caro uses the little Friedrich as a "device" to call up the Friedrich body. That's the best I can think of out of a bad situation.
(Frankly, I don't remember how specific they were with how Caro met Friedrich or anything else that might provide a hint or limit, so I can't say I'm sure this won't bump into anything, but ...)