I'm still unsure which ageny you are referring to.... I'll assume at this point it's OPP. ERT guys respond on whatever OPS channel they are assigned, which may get shifted over to a TAC channel at some point. You mentioned ERT, but I'm also wondering if you are referring to the TRU team. Again, they typically respond on whatever OPS or TAC channel has been assigned to the event, but will sometimes switch to their simplex encrypted channel when on scene (410.1875).
OPP has TRU teams and ERT teams. TRU is the big guns based out of certain locations in the province. ERT is basically in each area, their job is to contain an area and wait for TRU. They have a higher skill set, but don't pack the firepower that TRU does.