Это очень странно, я бы смотрел в логах сервера
https://www.3cx.com/docs/sip-trunk-options/ Может, ваш провайдер игнориhetn сообщение 600 Busy Everywhere?
Number of SIM Calls
Location: SIP Trunks → SIP Trunk → General tab → Trunk Details section
This option will allow you to restrict the number of calls being processed through a SIP Trunk. This number is usually the number of channels the SIP Trunk provider allows for the specific account.
The number of simultaneous calls per SIP Trunk is the sum of inbound and outbound calls at any given point in time that are using the specific SIP Trunk.
When the maximum number of simultaneous calls per Trunk has been reached, 3CX will not allow any more outgoing calls through this SIP Trunk.
At the same time, If an incoming call is received, 3CX will reject the call with a
“600 Busy Everywhere” error message, similar to the message received when the incoming calls are restricted.