A customer( connected to the ONT) is in call, when he receive another call ( double call) the caller don't receive the "ringbacktone" then it said that he is occupied but not forwarded to the msg vocal . The customer can take the second call. The problem is the second caller does not hear anything, no ringback tone, and cannot reach the voicemail.
IP Multimedia Core Network Subsystem (IMS) is an architectural framework for delivering IP multimedia services. It is needed to actually check the core equipments ( PBX equipments type).
After this requirement, we find out that the IMS says that the SIP 182 QUEUED message is not supported at the operator interconnect. The caller's network generates the Ring Back Tone only if we return a 180 Ringing message.
From the ONT webpage, please find 110=0 and modify it to 110=2.
The meaning of this parameter is as below:
control responds with 18x
0: Respond with 182 before responding with 180;
1: Respond with 183 before responding with 180;
Do not respond with 18X before responding with 180;
Do not respond with 18X before responding with 182;
Do not respond with 18X before responding with 183.
In order to apply the change to bulk ONT's, it is needed to access the U2000:
After the above suggestion, customer confirmed that the issue is solved, and ringback tone works properly.