As a slight (non-NHS, I guess) diversion:- in my experience "On-Call" can also be a contractual obligation (requirement) ... and - as has already been mentioned - private* patients (often paying OTT money) sometimes expect "premium service".
Those who have worked overseas may recall that "carrying the bleeper" was generally regarded by many as a "badge of office". I must admit that I didn't really see it that way myself, but nevertheless ended up burdened with the darned thing for extended periods on a number of occasions (as in, six months or more). And I can relate that - even at this distance now of twenty years and more ago - the "novelty" of being called in during the night every time they wanted to connect up a baby to the Bear Cub ventilator rapidly wears off.
*
Or VIP - both "so-called" and de facto.
