This should be of interest to the list.
I must credit the find to Schneier's blog: https://www.schneier.com/blog/archives/2013/12/operation_vula.html

"Talking to Vula" (http://www.anc.org.za/show.php?id=4693) is the story of a 1980s secret communications channel between black South African leaders and others living in exile in the UK. The system used encrypted text encoded into DTMF "touch tones" and transmitted from pay phones.