liam See if this works for you. Make a drop down/multiple choice field with the options: email, phone, both. Make it required. Make required phone and email fields and use logic to show one, the other, or both, based on the choice for the first field.
Making the first field required forces the user to show at least one of the email/phone field set. I'm deliberately being much more terse than usual due to lack of time right now, so let me know if you need me to elaborate.