UA flights West of ORD
can voice call dispatch on 131.050 and for flights East of ORD
131.400. Frequencies cover about 200-300nm around the Chicago area depending on altitude.
However like OPNLguy states much of the comms these days are via ACARS.
Also might want to listen to Arinc frequencies for airlines that do not have own radio networks or make use of ACARS.