PM will send text messages when you reach 75%, 95%, and 100% data limit.
The article referenced mentions 100% notification, but in my experience, I've never received such an SMS; only the 75% and 95%. At 100%, it just stops working!
You do realize that they already have the app fully developed. It's called "MyTelus". All they need to do to offer it to PM and Koodo customers is change the hex codes for colour, a couple of server addresses in the code, and the logos. About an hour's work for a developer.
Actually the old Koodo app was just that -- a re-skinned version of the MyTelus app. I was familiar with both. And Koodo abandoned the app in favour of a mobile-friendly website. Making the self-serve portal mobile-friendly would be a worthwhile investment for PM to pursue, since they've increased visibility/advertising, and are probably bringing in more customers.
Since Telus acquired PM back in the day, I would imagine that the PM backend and billing system might be different enough from Telus or Koodo that it's not just an hour's work. I could be wrong, but it's probably a case of needing to develop new APIs, interfacing with them, perform integration testing, etc.