I have been on call at two UCCs and they varied pretty significantly from one another.
The first includes those in the on-call rotation carrying a pager for one full week at a time and responding to after-hours crises. These may include pages from the university police department, area hospitals who have a student they're admitting or discharging, or a residence life director. The majority of the pages have to do with suicidal or homicidal ideation (with varying degrees of intent and plan), sexual assault, or bizarre thinking. The frequency of the pages also varies pretty significantly from 0 pages in a week to several, and may be addressed either over the phone or in-person, depending on the situation. This university also does not use ProtoCall or a similar system as a way of addressing or screening after-hours calls.
The second includes those in the on-call rotation carrying a university cell phone for one full week at a time and responding to after-hours crises that are initially screened by ProtoCall. As a result of ProtoCall, very few calls are actually transferred to the on-call cell phone (far less than at the first university I referenced). The reasons for calls are similar to those I previously mentioned, and also may be addressed over the phone or in-person, depending on the situation.
Hope this helps!