The connection between the telephone set and the ONT is abnormal.
The ONT port to which the telephone set is connected is configured incorrectly.
The telephone set does not register with the voice server.
The voice service of the telephone set is not configured with a high priority.
The line connections are abnormal.
The telephone set is faulty.
The numbers configured on the ONT(such as HG8245H) are incomplete.
The digitmap configuration is incorrect.
The codec and authentication configured on the ONT(such as HG8245A) are incorrect.
A phone number conflict occurs during the registration.
The voice IP address fails to be obtained.