Apologies for thread creep. Whilst flying in a Cessna 402 from Malakal to Khartoum in late 70's (I think 79) we were trying to raise the notorious Khartoum Tower to advise eta but struggling due to heat haze/limited (non-pressurised) ceiling.
Imagine our amazement to hear a cut-glass British accent offering to relay our call. It was a B.A. Concorde flight out of (iirc) Dar es Salaam on one of the experimental routes. As quick as a flash they had relayed and advised that Khartoum were aware (a result in itself). Made our day !