Call Drop After A Few Minutes

[idscan.net]

• 3CX Version, Professional Perpetual 16.0.5.612
• Server OS, Windows Server 2016
• Provisioning Method: Stun
• Has the Firewall Checker passed: YES
• Are custom Phone Templates being used: NO

We have several users on our system but only one person is experiencing this issue.
He will place a call and after a few minutes the customer cannot hear him nor can he hear the customer.

He is a remote user, and the majority of our users are in a separate office building.

So far we have tried factory resetting his phone,
buying him a brand new phone,
changing his router settings to allow the necessary ports to forward to the phone.

Where should I look to troubleshoot this?
It seems that no other user is having these issues so it doesn't seem to be on the server side, but we've factory reset the phone, re-provisioned, and replaced the phone entirely.

 

[SteveITS]

Is SIP ALG disabled in his router?
Same problem using the web client?

 

[leejor]

Does this happen on outside calls only? Outgoing, incoming, or both? Is this a STUN extension? Have you checked the 3CX Activity Log to see which end , ended the call?