Hi Khun Gunpong,
Good information/troubleshooting checks below.
Did you raise an incident with your local service desk? Any problem should have Incident raised and investigated by local team initially. Once initial investigation has been completed (like below checks) you can update the incident and refer the Incident to Network team.
In this case we will call out a ticket to BT SD and they can get BT voice team to investigate on their end.
If you have already raised an Incident please let me know the IM number so I can help to follow up with BT.