Easy. Pay them a decent salary for every hour they are on call. Simple as that.
The "easy" answer is to put it in the job ad, and hire people who agree to it. Because forcing that requirement on an existing employee just isn't going to happen.
* by all means, pay me an indecent wage, enough that I can retire after a year!
At the very least, make it opt-in, and keep raising the on-call pay until you have enough existing employees who will decide the trade off is worth it. At some point it maybe ends up being more economical to follow your suggestion to put out a job ad specifically for on-call people.
I'm with you. On-call is never worth it. If a company needs 24/7 coverage, they should hire round the clock.
I did on call 24/7/365 for 23 years. I'll never do it again. My phone goes silent in the evening and doesn't come online again until I get up to come to work. That's never going to change. I'm not 20 anymore, I don't function on no sleep.
I hope you made enough money to retire comfortably. That sounds like utter hell.