Having the PBX check if the caller is already on the phone has not worked well at least not the approach that is getting removed in this commit. It is not the right way to solve the issue. Its better for the phone endpoint to block the intercom or auto answer when it is already on a call.
One way to do this that worked in testing was to disable call waiting. Then the call is rejected and not allowed to interrupt. the call that already exists. Expect there are also other ways to instruct the phone not to interrupt active calls when it receives a SIP message to auto answer.
