The problem is that most current "HD" voice implementations seem to be incompatible; even if two carriers offer "HD" voice, a call between customers of those carriers won't use it.
Well, there are five distinct cellphone networks in Montreal (excepting the Bell/Telus tower sharing agreements), and that's ignoring the different budget-brands where Bell Canada owns Virgin Mobile, Rogers owns Chatr and Fido, Telus owns Koodo, etc... The chances of all of your friends being on the same network is zero.