I personally can see where Rogers is coming from re wifi calling being chargeable. When you make a call over the HSPA+/VoLTE network, you are using the tower to leverage what is theoretically an internet connection to Rogers' backend, which then routes the call and completes it. When you use wi-fi calling, your wi-fi connection is simply replacing the tower. You still have to leverage Rogers' backend systems to route and complete your call. So it isn't "totally free" for Rogers to provide the service. It still costs them money when you use their backend.
The reason RON probably didn't use minutes up was probably because they knew how much it actually sucked, whereas wi-fi calling is an easier, better integrated way of doing it. RON is also a poorly-done VOIP service, whereas wi-fi calling is of similar or better quality of sound to the cell towers. So better service = higher price point.
To solve the issue you could always switch to a plan with unlimited minutes. The pricing isn't that horrible if you ask to speak to client care/retentions, and with roam like home to sweeten the pot vs older grandfathered plans, it just might be worth it. Cheers
regardless to how you connect to the call you are making/getting ultimately that call is being routed through Rogers phone network and thus they are still billed for connecting calls to the party you are speaking with.
I'm running iOS 9.1 public beta on a 6+ (updated from all the previous Dev 9.0 Betas and running Rogers 21.1.5 as carrier settings) and Wifi Calling is working...as long as Rogers One Number is not activated on your account. My phone is actually on Wifi Rogers right now at work where I have only 2 bars of Rogers LTE coverage it seems to grab on automatically when it dips below 3 bars...the call quality is insane! i could hear my wife breathing and little things in the background that no cell call ever could before. I cant wait for VoLTE to come to iPhone now. I know it can support it with a carrier update. any update on that Rogers? i see the original LG phone is now at home on VoLTE with a bunch of Samsung phones now