Formal notion of a call graph for Turing machines
Turing machines have no concept of "calling".

Tou can jump to another state and that can look quite a lot like a procedure call, especially if an author's description of the machine in English uses words that make it sound that way. But that's all you can do. In particular, there's no notion of "return" in a Turing machine. There's no way to say "OK, this procedure is finished&nbsp;&ndash; now go back to wherever we came from." All you can do is say "OK, this procedure is finished&nbsp;&ndash; now go to this specific point."

Of course, because Turing machines are Turing powerful, you can simulate these things. For example, you can write some notion of a return address on the tape as part of your simulation of a procedure call. But that's not intrinsic to Turing machines: it's just a property of the particular Turing machine you're writing at the moment. A coding convention, if you like.