The best solution would be to throw away Smartphone S, install Skype on Computer C (which presumably has an internet connection via the Wifi you mentioned) and then initiate a Skype call to the phone line of the person who cannot attend the meeting.
All this requires is a set of speakers and a microphone on the computer, no other hardware. You would have needed these two items as a minimum if you went with your original plan anyway.
At worst all you need in this case is some credit on skype to call the person, if they can install Skype on their laptop or mobile then you would not need this credit at all.
Skype should be able to cancel the remote signal from being picked up by the microphone.