Not just in Africa...either
Have noticed this type of 'operation' only one time, while deadheading on the obs seat west of Medan.
Our aircraft, having reached FL350 about ten minutes before, was asked by Lumpur to descend to FL310 due to crossing traffic ahead.
The Captain 'politely refused'...Lumpur directed again, and the Captain refused once more and promptly switched off the HF.
Yep, there it was, passing ahead about three miles away, was the crossing traffic.
Have to wonder about the thinking process that goes on with these folks.