From the late 60s, COMSAT corporation provided all the broadcasts from space missions, including Apollo-Soyuz. COMSAT used its own network of INTELSAT satellites as well as NASA communications satellites, for the transmission of TV images around the world. The greatest achievement of the company was live broadcast of the Moon landing by Apollo 11, which brought together the largest television audience in history. In 1975, upon COMSAT information, ASTP handshake broadcast audience was more than one billion people (a quarter of the world's population that time).
ATS-6 - prototype of Satellite TV - For the first time NASA experimental satellite has been used for direct TV broadcasts. Space Voyage created a series of cachets canceled on the day of the ATS-6 launch, May 30, 1974, plus various ASTP cancellations.
In addition - following the revolution on Madagascar and loss of local NASA tracking station, ATS-6 took tracking and monitoring functions.
OTC – Overseas Telecommunication Commission is array of facilities that from beginning of the 20th century were used for radio communications with ships and later aircraft. I don’t know what role did they play in space program but there are some ASTP covers from these stations.