I am not from this era so would know the system then. But there was a guy in a FB page I was in around the time of the ITV series airing that's also when I joined that group. He worked for BT, not an engineer but biling dept and as per him all ISD calls were itemised in 1985 so could be traced but for local calls it was not itemised so couldn't be traced, only the minutes used will be clocked and charged. I think he could be right that's why Mugford & Co did not have outgoing call facility, as students they could have run a huge bill and problem for the landlord if they refused to pay or dispute who made the most calls when it's time to pay bill.
Anyways if it existed Defence would have dug it out. And if it did get traced there was a call from WFH to Bamber's cottage at the said time. lmo, Bamber wouldnt be where he is now.