Availability vs OnCall in IT

In my organization, IT is at a crossroads with regards to after hours issues. The crux of the matter is in the subject: Availability vs being OnCall.

The difference for this discussion is OnCall carries the pager/cell phone and is expected to respond to any issue. This is usually a scheduled responsibility - 1 week a month for example. Availability is a subject matter expert (SME) being available if there is a failure in a system they are responsible for. This is usually always, but never used outside specificly identified incidents.

OnCall is expected to spend their assigned nights/weekends sober with no plans. Availability is only activated when others have triaged an incident down to the SMEs responsible system but could be anytime.

First, renumeration. Is OnCall or just being available built into the salary of an FTE? Should renumeration be monetary or comp time spent the week after being OnCall? Is there an expectation of anything after hours built into the IT industry as a whole?

Second, responsibility. How can you find ways of sharing the load? Usually you don't have many specific SMEs in any given department - so what is important to share to others for assistance? How can you get others outside of a specific IT discipline to engage or even participate in an OnCall rotation? Where do reaponding to automated alerts/notifications - most which are transitory or red herrings - enter the conversation?

Context: I've been in sysadmin, NetOps, infrastructure type support position a majority of my career. In the 1990-2000s, there always felt like a requirement for unpaid after hours work regarding what I supported - but not being an after hours helpline. Now that I'm directing several of these same positions, I'm trying to determine how to be fair to the individuals, fair to the team, and to stretch whatever options I have within my organization.

Note: conversations about after hours support can get heated. Don't beat me up too much - I'm just trying to be as fair and transparent as I can be

Thanks!