Yep! Basically, you'd point out that it's not economically-viable for you to provide a small number of high-value, low-revenue on-call hours on demand, so they'

Yep! Basically, you'd point out that it's not economically-viable for you to provide a small number of high-value, low-revenue on-call hours on demand, so they'll need to reserve some time each month to make sure you're available when needed. (It's a bit like the old joke: "Ten cents for tapping with the hammer, ten thousand for knowing where to tap with the hammer.") If you've done BigCo consulting, it's essentially an extended hypercare scenario.