In a tv script, how do I write a phone call that cuts back and forth between the two characters? Do I constantly write "CUT TO" after each line of dialogue?

I don't know about a TV script, but in screenplay format you'd insert an INTERCUT TELEPHONE CONVERSATION and then proceed with the dialogue as usual.

that would've been a useful formatting slug for me to have figured out a long time ago

The best thing to do would be to find a film/show that has an example of what you're thinking of and try to get your hands on the screenplay (places such as Drews Script-o-rama might have what you're after).
There's no better way than to learn from the real thing.
However if it's something you will be directing yourself then I wouldnt matter too much about the little things like that, you could technically just write whatever you want.