I know what can cause this problem, but I don't know what resolves it.
The problem can occur if you delete voicemail at
https://www.freephoneline.ca/mailboxPossibly doing so causes some account sync bug to occur.
After deleting all voicemail at
https://www.freephoneline.ca/mailbox, receiving another voicemail, and then deleting it by dialing *98 or my Freephoneline number, the problem occurs.
Freephoneline's server will eventually send a packet to me, triggering MessageWaiting despite no voicemail being listed at
https://www.freephoneline.ca/mailboxBoth times I've experienced this problem over the last two years, the trigger was deleting voicemail at
https://www.freephoneline.ca/mailboxI do have "voicemail to email" set to "copy" at
https://www.freephoneline.ca/voicemailSettings (I don't know if that setting has to be enabled in order to reproduce this problem).
I also shut my devices (including router and modem) down for two hours. Changed DNS servers. Changed proxyserver. Nothing helped.