First option: Move your number to a Page Plus Approved device. Submit a ticket, it will get investigated.
Second option: If you don't want to do that, wait until the LAST day of your current TNT1200 plan, call them, have them remove it completely. Reprogram your phone using *22890 SEND, then, after waiting a few hours WITH YOUR BATTERY OUT, add a new TnT1200 PIN to have it rebundle a new plan on your line. The outgoing balance notification gets turned off during the plan bundling process. Most likely that part of the bundling process got stuck the first time you bundled the plan onto your line.
It's up to you whether you use cash PINs or stacked PINs, however, Page Plus will only fix billing problems on lines that use Stacked PINS (generally speaking) because the older cash method is too easy to "mess up" on and cause a problem with.
I would probably try the second option I wrote above.