forms - Combining email and phone inputs into a single text field

Let's say you want to combine both email and phone input into a single text field because you require either or, not both. What's the best way would you approach this? As far as making it clear without a shadow of a doubt the user can enter an email address or a phone number. Also, how would you deal with international phone numbers? Ask the user to enter their country code first?

UPDATE: Please think about this with and without JavaScript being present. Don't get so hung up on validations. Let's convey to the user what we're doing first.