Every system is different, I used to monitor a system that had heavy I-call usage as well as phonepatches. That was the mid-late 90s so cell phones were not as common for heavy usage as they are now.
As more and more TGs are assigned to trunked radio systems, then it would be likely to see I-calls severely limited to only certain users to save on bandwidth.
Case in point......This particular local system carried the highway patrol traffic as well, but the regular road troopers (below SGT) did not have the ability to initiate an I-call, except one. He had his radio reprogrammed at the local radio shop so he was able to make I-calls to county officers as well as his fellow troopers. I remember one night where him and another trooper were talking for almost an hour, my boss (a ham like me) joked with me that he talked so much he should have gotten his ham radio license :lol:
At some point someone spilled the beans, and his radio was reprogrammed back to it's intended operation. Was a shame too, he always had rather lively conversations