Is my statement correct?
As far as it goes, yes.
But having somebody pick up a phone is not "service", it's what happens afterwards that matters and what drives the columns on that graph.
IDNet don't run a routine customer service outside office hours, so queries about billing etc will have to wait. But they do
monitor the phone messages, and when someone has a major connection problem they'll get on to it as soon as they become aware of it.
If it's a problem within their own network that they can fix, they will, if it's a problem with BT's equipment they'll liaise with BT to get it fixed whenever BT can get around to it.
Which last may explain the relative height of the BT columns on that graph