I don't consider them equivalent at all, though embarrassingly I don't know enough about wireless bandwidth and RF comms provisioning generally to be able to state conclusively that they're not.
Many people have unlimited or some high limit but don’t use it all.
(It’s not entirely fair as the device can do things like text and phone calls, but if I consider my phone as a phone only I was paying $100 for about 5-10 minutes of talk time a month (used) so in a way I was spending $10 a minute. Of course the main thing people want is data. We’re probably a few years from phones with data only and no phone number at all