VoIP Configuration Problem on Device HG8245H

In the Status> VoIP Information menu, the phone number is correctly configured but on Call Status it is Disconnecting.
When we click on the "Restart VoIP" button, initially returns to Call Status: "Idle" but immediately after a new call returns to "Disconnecting".
We have configured the numbering on port no.2 too, but the problem remains and the customer tried with 2 different phones connected but the problem is found on both.