From a plan point of view, probably better to go with the Your Choice $30 BYOD plan if you're already using this plan type, gives them 300 local mins and SMS for $5 cheaper. You can use 3rd party apps / device settings to control what other access th...
Unfortunately Telus does not offer a subsidy/financing option for the remaining $3-400 cost of the new iPhone and you'd have to pay it out of pocket, even if they did it would increase your monthly bill by $15-20. You can always buy a refurbished iPh...
You can also check what app or service used the data via your settings (Settings -> Cellular -> Cellular Data Usage section). What Nighthawk suggested is the most likely, also make sure apps that download large updates are set to do so only over WiFi...