I’m not sure if this is what mueschel is getting at, but the original proposal stemmed from a restaurant with an SMS-only phone number. Ironically, sms=yes|no|only is in partial use, to indicate whether SMS messages can be used. So to say phone=no would kind of make sense to mean that a phone number is not for voice calls.

In the end, I think it made more sense to formalize sms=number instead.