Note:
1. If C does not answer the call, A can back to continue the conversation with B;
2. Once A hangs up the call, the conference is ended, while if B or C drops the call,
A&C or A&B conversation continues.
3. The conference feature is not available on all servers. For more information,
contact your system administrator.
4. To realize the 5-way conference, the line should be all available.