Contacts Lost on Poly VVX-411 After Reboot

We are new Zoom Phone users.  Things are generally OK but users are complaining that contacts that they enter into the phones are randomly lost.  
Have tried researching the issue and saw a possibly 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.