It's not in Apple (or any phone maker) control today. With current technology you pipe 8 kHz voice to the wireless modem and the encoding is done there, following strictly defined cellular standards. So it's in the operator control.

Apple has extra leverage, though, because of its perceived importance in the market. Look at visual voicemail: Apple decided IVR voicemail sucked, and worked its way through carriers until it could find one that would support changing its voicemail architecture. That carrier then turned both the iPhone and that feature into a competitive advantage.

If Apple really wanted to, it seems like they could have strong-armed their way into some higher-quality audio standard -- earlier deployment of VoLTE if nothing else, since that's going to happen eventually anyway. That said, I'm not especially surprised that they didn't; Apple has historically actually been pretty timid with wireless technology, and the biggest bafflingly absent feature of the original iPhone was that it didn't include a 3G radio even though 3G was already widely deployed.

There are limits to Apple's leverage in the mobile marketplace. When the original iPhone was introduced, they got AT&T to implement Visual Voicemail and ship the phone without a carrier branding logo, but not much else. They couldn't even get AT&T to agree to ship the phones without a subsidy lock, despite the fact that there was no subsidy for the first few months (nor could you legitimately unlock an iPhone associated with AT&T ever until earlier this year).

At this point, analysts and investors expect Apple to ship over 5 million iPhones in the first weekend of availability. Resistance by a single carrier (there are 3 now, and that's just in the USA) will seriously impact that number. Moreover, carriers have other serious challengers at their disposal and are pushing them hard (e.g. Samsung Galaxy S3).

Asking a carrier to promote a phone is one thing. Demanding they overhaul their entire network is probably asking too much.

Exactly. They didn't even need VoLTE; higher quality voice has been possible on 3G networks for years. If you have an HD-voice capable phone, then I believe you should get HD calls if you use Orange (now Everything Everywhere) in the UK for instance.

Lots of modern smartphones support it, but it's not always turned on apparently.

Apple controlled the choice of hardware in the iPhone and could easily have picked a modem that came with AMR-WB codec support. It's not as if the codec has onerous hardware requirements.

Yes, putting it in the original iPhone would probably have been a feature too far, but it could have been added to any iPhone from the 3G onwards as I understand things. It's exactly the kind of premium feature that you'd think Apple would have loved to have had associated with the iPhone.

Whether they could force the operators to upgrade their networks is another question though.