Yeah, it's not possible (or perhaps just a really, really bad idea) to do "always on" oncall with that kind of SLA. Either people will be constantly missing it, or you'll burn them out really fast. The SRE book talks about this, I think, in terms of the minimum number of people and distributed offices you need in order to do a robust oncall rotation.

Ah I see, thanks. Hopefully you don't need to use the bathroom too badly during that shift I guess :)

Then you transfer to secondary or nack the page for the secondary to handle.