Ask HN: Retainer and rate question for software engineer

I'm curious how a contracting software engineer would structure an agreement for work where there may or may not be time spent actually doing work. Simply put, it's an "on call" type arrangement for server administration and hotfix/bug fixes if the need does arrise.

I've contracted before, and have my standard rate, but have never been in a situation where I'm on call, but have the potentiality to not do any work what-so-ever.

What is the industry standard for this sort of arrangement? Thanks!

I'm also sorry if there's a similar ASK HN posting. I perused google and HN a bit and didn't find any similar posts.