I'm making my first form and am asking for the user's phone number and e-mail address, so that we can get back to them with answers to their questions. We obviously need them to provide one or the other of those pieces of information, but we don't want to FORCE them to provide both, and we don't want to impose on them which of them we want them to provide. Is there a way to just make sure they provide one or the other, but not REQUIRE either of them specifically?

No, there is no way to do that. It really is just one way or the other on what you want to do there.

why not set the email address to REQUIRED but not the phone number

Add a tick box for preferred reply from yourself so they can tell you to phone not email