Pretty normal for government work. My dad was a plumber for LA County, on call pretty frequently to respond to broken pipe emergencies and what not, and got paid an hourly rate on top of normal salary for that, whether he got called in or not. Even as a software developer contractor, when I've worked government contracts, if you're authorized to charge overtime, you do have to actually charge it, and get paid more than your salary, if you work overtime. Otherwise, you're donating labor to an executive agency, which violates the Constitution's granting of exclusive power of the purse to Congress. This is the same basic reason that, if you donate money to the government, you can only donate to the Treasury general fund and not to a specific activity, also why Iran-Contra was illegal (one of many reasons, I suppose).

It is common in UK to be salaried but on call on an hourly rate.

Right, and in my situations, if the crisis that demanded I be brought in was solved, it was assumed/expected I'd be taking those hours out of my normal work hours. So maybe leaving early one day, or an entire day depending on the severity. On call hours were treated as a multiple vs one to one.

Seemed pretty sane/rational to me that I never minded being on-call. There were the occassion where I was not immediately available, but I notified everyone when I would be and got to work when I said I would.

I can see where this is ripe for abuse by either side, so I'm not trying to brush off those that are getting abused by it.