I am all for flexibility -- but I think the quid-quo-pro here is that the phone is only for legitimate problems.
Not for example: "Oh the customer has decided that the feature we thought we implemented wasn't what he wanted. Can we have this weeks' requirement met by Monday?"
Honestly, I'm talking about problems that I'm sure are actually affecting the core business. Anything trivial I'll deflect long before I talk to an engineer
This is all something of a moot point. Looking back through my diary, I've only ever had to call our IT support out of hours, never one of my own engineers. Someone on my team has always independently began to act on whatever alert has been raised.
Not for example: "Oh the customer has decided that the feature we thought we implemented wasn't what he wanted. Can we have this weeks' requirement met by Monday?"