Last week, Verizon quietly announced it had launched Voice over LTE (VoLTE) service. The event marks the availability of HD voice services on all four major U.S. carriers and may be the service with the most heartburn attached. Also, there's still a long way to go before HD voice is ubiquitous, with carriers yet to launch network interoperability for calls.

Verizon's HD voice service, dubbed "Advanced Calling 1.0" (AC 1.0) by the carrier, bundles in point-to-point video chat and six-way calling to other capable phones. As of last week's announcement, the only phones supporting VoLTE and AC 1.0 are the Samsung Galaxy S5, LG G2, and the two new Apple iPhone 6 models. Verizon hasn't made the process seamless, requiring users to verify eligibility, then log in online to "Change Features" and add "Advanced Calling 1.0" to the phone. Once that is done, you have to reboot your phone and reset call forwarding.

When you turn on Advanced Calling 1.0, ringback tones, caller name ID and company name ID are "considered incompatible."   Yes, it appears you lose caller ID data if you turn on HD voice, which may explain why Verizon may be soft pedaling the launch. There's also the interesting issue that Verizon won't charge for those services if they are turned off, making it possible the first carrier that may actually lose money on HD voice.  Verizon says those features must be "upgraded" to work on AC 1.0 lines, but gives no timetable. 

Unlike AT&T and T-Mobile US, Verizon says AC 1.0 calls can't be seamlessly switched between 3G and 4G networks, possibly explaining why the carrier allows VoLTE to be toggled on and off from the phone once the service is turned on. If you start a call on a 3G network and move into a 4G LTE area, the call continues as a narrowband CDMA call. Start a call in 4G LTE and move into a 3G area, and the call drops.

“If you’re in an area with a weak 4G LTE signal, you may want to consider temporarily turning off the VoLTE Call setting on your phone to avoid dropped calls," recommends Verizon.

For AT&T or T-Mobile, moving between networks isn't a problem with calls automatically transferred between 4G to 3G service without the user being dropped. While not explicitly stated on AT&T's FAQ, AT&T callers are likely "downshifted" from HD voice to narrowband when service moves from LTE to 3G networks. AT&T lists ringback tones, Smart Limits, Office Direct and Office Reach services as incompatible with its VoLTE service. T-Mobile users should have the best of both worlds, since the company has HD voice turned up on both its 3G and LTE networks, avoiding the "downshift to narrowband" problem.

Verizon callers undeterred by all the hurdles will know that they're on an HD voice call because they'll have an "HD" logo appear on the upper right corner of the in-call screen.

Sprint provides an "HD voice" logo on some of its phones when an HD voice call is established. But Sprint's version of HD voice is wired to its legacy 3G CDMA network and isn't directly compatible with moving HD calls between either LTE networks or existing 3G HSPA networks. Sprint will either have to transcode between its 3G CDMA HD voice format and VoLTE at some point in time, but the company is in no hurry to roll out VoLTE. Given the headaches Verizon has encountered in bringing up baseline VoLTE in parallel with its existing CDMA network, it might be taking the wisest short-term course.

The next great hurdle will be HD voice interoperability between carriers. VoLTE calls should be relatively simple, but it all depends on the different tweaks and interpretations individual carriers have made in implementing VoLTE. Session border controllers (SBCs) will likely play a key role in smoothing the rough edges between VoLTE interoperability.