"Obviously the client doesn't want to pay $$$s to retain me/enough to hire a 24/7 support team" Then they can't afford what they want. It's as simple as that.

"Obviously the client doesn't want to pay $$$s to retain me/enough to hire a 24/7 support team" Then they can't afford what they want. It's as simple as that. Even if they can afford it though, define the terms as strictly as possible, and set a cap on all liability.