Right now it's hard to say as the Bring it Back amounts & pricing for the new iPhones are not available. However once they are with the Bring it Back program whatever the initial Bring it Back value that is taken off when you purchase the phone would be forgiven so for example using todays phones if you were to pick up an iPhone Xs and returned it after 12 months they would forgive the up to $300 that is initially taken off but would still need to pay any remaining financing amounts on the device.
For any Bring it Back fees it would just be if you decided to keep the device instead of returning it before the end of you term and in that case you would be required to pay back the initial Bring it Back amount applied.
Your best option would be to call Telus directly and ask. I don't think there are many mobility employees around here.