Technically there is no requirement for a mobile app or data.
Through PayByPhone you can pay for parking using SMS, by calling (IVR), a web app or mobile apps.
But as the author points out, it's hard for the user to know about all the options.
That's why I'm personally not a fan of cities that allow more than one vendor. How to advertise all of them on sign posts and stickers (sometimes given different location IDs that you need to use with each vendor) without overwhelming the user?
Note: I work at PBP