iPhone 5 HD audio ?

[avmsam]

so i know at the keynote they said the iPhone 5 will support HD audio calling but only certain carriers support it so far. I've read that Telus and Bell both do and Rogers says they're working on it. anyway, my question is do you need to enable this on the phone ? do both people on the call have to be using a phone/network to support it ? i just wanna experience it and compare it to normal calling.