Call Lenth vs Call Cost
I did a lot of work on this in the 1970s, (not in the UK) automating the production of average call duration distribution data when calls were metered rather than computer recorded.
My studies found that the more expensive the call the longer it lasted, this rule was across all call classes, even international.
During down turns the pattern was even more marked and it became easier to see what happened.
Un-metered calls (or calls that were part of a package) were a don't care element. If you wanted information you made a call, sometimes for a few seconds hung up and called again later.
If times were tight, you made a list of items, then made a call, often towards the end of the day and asked all that you wanted to ask before getting the information and hanging up.
Traffic was often crammed into a shorter period of time towards the end of the international overlap causing the busy period to be far busier and later than normal. It played hell with the economics of dimensioning back then so congestion often resulted. Perversely, when things improved, we had more paid time and less congestion as traffic was better spread out.