Phone call routing to FB Messenger audio conference call

The background of the problem:
-      A person in a remote location can use a telephone, mobile phone, but is anti-technic, meaning does not know how to use FB Messenger even if we make an account for him. Is also unable to install any other app on his mobile phone, not to mention further steps under guidance to make settings… forget about it.
-      He is an important person, concerning different aspects, and we would like him to join us in a conference audio call.
-      Most of us are on FB Messenger and we already had a conference call last week.
-      Now we need this new person, who is able only to receive phone calls, to join our future conference audio call on FB Messenger.
-      The persons from our team doe no want to use additional conference audio call software/apps on their mobiles/PCs. All of them want to stay on FB Messenger. That make the situation harder to be solved.
-      I can use an additional mobile phone, laptop, software, app on my devices if is needed to solve the problem.

The needed help:
-      How can I connect, re-route, divert the phone call from that new person to our FB Messenger audio conference call?
-      I am open to any wires, software that I can install by myself, on my own devices.