If you are begging your engineers to be on call, you have built the wrong engineering culture. Your core engineering team should feel ownership of the app, it should be their baby. What happens in this case is EVERYONE shows up when there is a site issue, and your best engineers burnt out. Instituting an on-call schedule is what a good engineering manager does to force engineers to NOT be on-call 24/7, and instead institute an equitable distribution of responsibility. I think all small eng teams should have on-call rotations, and it's the reason behind why I built https://heiioncall.com/.