Eirene,
you could try the Highland Archive Centre Inverness on the following link who might be able to help,
https://www.highlifehighland.com/highland-archive-centre/
another place might be the Highlanders Museum on the following link, who cover the Cameron and Seaforth Highlanders which amalgamated in 1961 to form the Queens Own Highlanders. The Camerons and Seaforth recruited mainly from the highlands and islands so there may have been some of the Mackay family in these regiments.
http://www.thehighlandersmuseum.com/
Just a coincidence I started work for British Telecom from 1975 and knew another linesman called James Mackay who was based in Inverness but he retired in the 80's.