Local Contacts Lost from Poly VVX-411 after Reboot

We are new Zoom Phone users.  Things are generally OK but users are complaining that contacts that are entered into the Poly VVX-411 phones are randomly lost.  That is to say all local contacts are completely wiped out unexpectedly and seemingly randomly. 

I assume that is because the phones are periodically restarting and they are not sync'd with the Zoom server, but it certainly could be something else. 
Have tried researching the issue and saw a possible cause could be a blocked port on our firewall  preventing the contacts from syncing with Zoom - but no details on what port to look for.

Any suggestions on how to troubleshoot is greatly appreciated.

Thank you.