I think if companies need to pay with "days off" instead of "money" they would be much more carefull with on-call and have a much greater incentive to make the on-call - not call, proper procedure, taking care of on-call incidents so they don't repeat. They would empower developers to make sure to minimize it so they don't have a penalty of dev-on-vacation. when you have an oncall with fixed payment per hours, you just don't have enough incentive to minimize the effect, you have those people handling it on on-call payment.