6 (SS6) was introduced in the 1970s as an early common channel signalling method for telecommunication trunks between international switching centers (ISCs).
In some cases these were advantaged or multiplied with time-assignment speech interpolation (TASI) or digital circuit multiplication equipment (DCME).
It was of course practice to have standby route over a diverse path in case of outage on the original DP data circuit link.
CCITT No 6 was a method developed & implemented by administrations in a limited number of countries for use in correspondent International Switching of analogue telephone calls.
The next development was correspondent and non-correspondent switching of telephone calls by new operators in the countries where more sophisticated methods were required.