The road to HD Voice on mobile phones is a bumpy one

To make an HD voice call, you need to meet all of the following stipulations.

• Your phone needs to be HD-capable. Not just HD capable, but support the HD-Voice codec used by your carrier. In the case of T-Mobile, that means the iPhone, the Samsung Galaxy S 3, the HTC One S and probably most newer generation smartphones. For Sprint, that does not include the iPhone because Apple isn’t supporting the CDMA HD-Voice codec, but it does include the HTC Evo 4G.
• The phone you’re calling needs to HD-capable. Not only does the recipient need an HD device, it needs to be running on another HD-compatible network using the same HD technology as your device. Even if a Sprint and a T-Mobile customers both have the right phones, they can’t make HD calls to one another. If either customer called any other carrier or any wireline number, those voice connections also would revert to “standard-definition.”
• Both phones need to be connected to an HD-capable base station. Just because a carrier supports HD-voice doesn’t mean it supports it in all places. Sprint, for instance is enabling it as it upgrades its CDMA systems as part of its Network Vision overhaul (basically everywhere it offers LTE). When Verizon and AT&T launch their voice-over-LTE (VoLTE) services, both caller and recipient will have to be on LTE networks for the conversation to transmit in HD. T-Mobile, however, appears to upgraded its entire network to support HD.