I've only ever called in fatigued once, about 2am, after tossing & turning in bed for hours. No questions asked, just told by Ops to call when I was up, and they'd pax me home. All very civilised I thought.
However, colleagues at the same company have told of feeling some to significant pressure to accept duty when they've made a similar call. I suspect it's a combination of how one notifies Ops and the sense of the Ops controller taking the call.