Once you ported and no longer have any remaining lines, the account is closed while with the MyVerizon app, it will prompt only for Phone Number and not username as there is no longer a option to login by username in the newest version which one has to be forced update to, it will not accept the phone number saying it does not match their records. Basically, even a older version of the MyVerizon app won't work as it will have a screen with only a option to update to the newest version and no other options.
You can only access VerizonWireless.com and get to MyVerizon over there which claims to be temporarily unavailable but everything is gone including the SmartRewards and the balance will show $0.00 instead of the actual balance which is available on the automated phone system while you can still make Payments in the Pay Bill section, View billing statements for the last 18 statements, My Cloud Media and Contacts which doesn't work as seen in the attached screenshots to explain it better. You will still be able to access documents and the Payment history.
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As for the phone system, if you call the 1-800-922-0204 during hours when the representatives are available, it will say your phone number is not recognized but there will be a option that says Making payments on a recently disconnected account and you can enter either your account number or social security number and then it gives you the automated payment system where it does give you the current balance and you can still make payments. If you call when there are no customer service representatives, it will say your phone number is not recognized and Good Bye and hangs up on you without passing you to the automated phone system. This is from my own experience after porting all my 2 lines to XFinity Mobile on September 9, 2017. My primary line took 3 attempts to port on the same day as the 2nd line which took 5 minutes on a 2 line family plan and did not happen until after I contacted Verizon Wireless by chat online while talking to XFinity Mobile online that took nearly an hour, probably because they didn't want to lose a customer since January 6, 1995 which is over 22 years from the days when they were known as GTE MobileNet and later GTE Wireless before the BellAtlantic/GTE merger that formed Verizon Wireless. A few weeks before I actually ported, I asked Verizon Wireless if I can still make payments on the phone system and MyVerizon app and they said the file will be closed so even Customer Service will not have access to the information but I would still have access to making payments on the automated system and the MyVerizon app. I guess the only thing is I have no access with the MyVerizon app, and only able to make payments on the automated system when they are still opened and not 24/7 like I did before.