Please submit a ticket so we can look into it. However -- with this said, please ensure that your customer has all of the appropriate router configurations - port forwarding, etc.

One example scenario is that a user may setup their device and immediately have success with incoming and outgoing calls, however fail to setup port forwarding.

When the success is a pure coincidence of the port negotiations at this point in time being allowed through your router, while these negotiations can and will change from call to call.

Thus effectively you will need to ensure that the traffic is making it to the end users device and vice versa.