You do have to have a phone number to send an SMS/MMS message from your Mac, which you get from the iPhone associated with your Apple ID. But just to be clear, that phone does not have to be present or even turned on to send and receive SMS/MMS texts on the Mac or a non-cellular iPad. The phone's only participation is providing the telephone number of an SMS/MMS enabled device. The telephone network connection is all handled by Apple via the Mac's internet connection. I don't know the actual mechanics of the process, but I know it works.
