Safety call recogniser anon here. I haven't thought about safety calls in a while, but when I used to live with a dear darling very kinky friend we had a sort of standing arrangement. If she was visiting a new 'friend' that she wasn't sure of yet and hadn't called by 8pm, I should call her. If there was no response something untoward had happened (or her new friend wasn't letting her use a safety measure she had mentioned to them, which is also untoward) and I should call the police or her Dad.

Luckily, I never did have to call her Dad and say 'Hello Mr L, your daughter is having painful things put inside her for recreational purposes but it's gone wrong, can you please go to this address?', but now I think of it the whole safety call concept is a fic goldmine.