Re: Ah, mandatory minimum callout times...
I implemented a four hour minimum for on-site visits in (roughly) 1990, a couple years after I went solo. Double on weekends/holidays. A few clients balked at the new rate ... I simply told 'em "Don't call me unless you actually need me".
A new issue arose. Convincing 'em to pay 4 hours (or more) for a one minute visit. The old TV repairman's maxim applied, "I'm not charging you for thumping your telly with a screwdriver. I'm charging you for knowing where and how hard to thump your telly, and for showing up to do it". The explanation seems to have worked ... although about a year and a half ago, a child CEO wondered why I'd need to thump a telly.