From my limited knowledge of just being a hobbyist for the last 15 yrs. and not being a professional, here's what I know..
"I-Call" is essentially a from of "radio-to radio" communications..i.e. similar to Nextel usage.
For example, the officer in Car 15 w/ radio ID #3 can punch up on his radio to speak to radio w/ ID #6, which is the officer in Car 18. This allows them to have a private conversation between the two of them, while not tying up a Dispatch or Tac channel.
While many depts. use Nextels, many still do not. This is a way to get a Nextel type usage without having the phone, better yet not paying for the service per phone.
This is a minimalist explanation at best, but gives you the idea.
Side Note On a Moto 9600 baud digital system, the scanners available cannot decode I-call traffic. They can, however, decode I-calls on an analog EDACS or Moto system.
Mark