In the past, AppleIDs could be created with only email address, and not phone numbers. I see that the current account creation step requires a phone number for verification via SMS/phone call.

That said, I don't believe you need to use that phone number for messaging – an email address is used as the contact information when messaging with iMessage. This is especially important for iPads/iPod Touches and Macs, which most don't have a cell phone number.