I just spoke to a friend regarding your issue, they said at their company they ran into a similar situation. Their solution was to use Zoom Video Conferencing,

I just spoke to a friend regarding your issue, they said at their company they ran into a similar situation. Their solution was to use Zoom Video Conferencing, apparently they have the option to have users phone in or have Zoom call users over the telecom network for audio whilst receiving video over the internet. I didn't ask on costs, but it might be a viable solution. Edit>> I see you tried zoom, did you try the telecom audio solution?