The My Fios app is crap. 1. Sending feed back gives you "you can't send feedback now, try again later" losing all the feedback you typed. Chatting with support tells you to leave feedback in the app. "Catch-22".
Real issue: Voice mail in My Fios app often mysteriously reappears the next day after being deleted. Plus many times it can't even find the voice mail at all, yet the stutter dial tone is there, and killing the app on my iPhone and restarting it, and magically it finds it. (Yes, I did pull down to rescan for voice mail). Also often it says, "It cannot connect to the voice mail system, it's busy" or "Cannot connect to the internet" and yes, it can.
The app handles error conditions, timeouts, and retrys badly, especially to back end systems.
If the app gives you issues then just call in directly from the iPhone. Call 888-234-6786 then you'll be prompted to input your phone number then # then your pin. When added the number in contacts you can also add a "wait" for your phone number and pin. This way you don't have to type it every time.