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).
There would be no need for filtering signalling away from speech, and the duty cycle of the costly trunk for each revenue earning call would be better.
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.