I am in the same boat. The explanations above are incorrect regarding taxing the full retail amount. At least according to what's shown on the mybell site. Bear with me...
If you click the "?" on the device purchase item on the webpage for your bill, you'll see a breakdown of the device agreement. It shows the taxed amount is on the reduced price after your agreement credit, not the full retail price. Then the financed amount plus the device return option adds up to the total reduced price (which includes tax). So the tax is included in the financed amount. So you will be paying the taxes off as you pay your monthly bills.
If you break down your bill further, you'll see that in "other charges & credits", it'll show the full retail (untaxed) price due then the financed amount, price reduction, etc as credits. Because the tax is baked in to the financed amount, you end up getting credited more than the full price of the device. If you look closely, you'll notice that you have a credit equal to the amount of tax on the reduced price of the device.
Than, further down in your bill, with zero explanation, that amount you were credited gets added back in as HST/GST/whatever. So it technically evens out. Bell is essentially crediting you tax in one part of the bill, and charging it again at the end of the bill. However, it makes the bill look absolutely incorrect since your subtotal was decreased by the taxed amount and your tax was increased by that amount. So you could easily end up with something like 150$ subtotal + 120$ tax.
tldr; Bell seems to bake the tax into the "financed amount", then for "reasons", they essentially credit you the tax in "One-time charges & credits", then charge it back in the GST/HST/whatever part of the bill. The subtotal and tax part of your bill then looks wildly incorrect.
I have no idea why it's so unintuitive and was able to get 0 explanation from Bell as of yet. I can't even confirm if this is right, but the math works on my bill.
