Unless the film was supposed to be 20 years ago the possibility that it could be something other than Illinois Bell or Ameritech is very real. I was in MSP (Minneapolis/St. Paul airport) and all throughout the airport the pay phones were Ameritech when the local provider in the twin cities is Qwest. So it is possible that Verizon could be a "COCOT" contracted by Wrigley Field or the property owner.
That said many film companies aren't really very good about telephone "authenticity" when you'll see someone depositing money into a single slot "fortress" pay phone and you'll hear "ding-ding" when they insert coins (ding-ding went out when three slot payphones went out.) Or when you see an older film showing a 'candlestick' or old WECO 302 have the ringing sound of a WECO 500 or showing a British series and they use American telephones. The assumption is that people do not notice these inaccuracies (except us telecom nerds of course!)