Yeastar made a decision to remove the call waiting function from the PBX, which, by the way, is a gross mistake. Instead they rely upon the phone to respond to the INVITE sent to let the system know if it is capable.
When testing with a supported Fanvil X7 with multiple lines set to the same extension I am able to call out any/all lines at the same time, the inbound ability to handle multiple simultaneous calls is not functioning.
I can busy out a line to an account and then call from another extension to the same extension/account that is busy, but the PBX never sends the INVITE according to the pcap trace. The PBX instead responds with a busy as I do not VM enabled while testing. As a result, it does not seem possible to accommodate more than one inbound call using the X7. I cannot attest to this being the case with other phones....yet.