Hey @Aminoo, we'd love to help clear things up! 
If you're on a BYOP plan and wish to take a phone on agreement you will have to change plan to a Small or higher plan (they range from Small to XXL). The tier you take depends on how much you'd like to pay up-front for your phone. For the full range of plans check out Fido.ca, the same applies for phones and their pricing
That said, the phone is yours to keep at all times. If you choose to terminate the agreement before the 24 months is up you'll pay off what you owe on the phone. For example, if you cancel halfway through and got a $240 credit on the phone, you'd have to pay $120.
If you stick around for the entire 2 years then the agreement ends and you owe nothing. The phone is still yours to keep In terms of the plan, it won't change. You can at that point decide if you want to change to a BYOP plan or renew your agreement with a new phone and keep your plan.
Another thing to note is that you can change plan at any time during your agreement at no cost as long as you stick to the same plan category (i.e Small).
I hope that information helps! If anything is unclear, let us know.
