Verizon users rejoice! The unlocked phone floodgates may soon open very soon.

After an upcoming software update, the HTC One A9 will be the first phone without CDMA that can supposedly make voice calls on the Verizon Wireless network. While the carrier has had some LTE-only tablets and hotspots in the past (the Ellipsis series), until now it has required support for its CDMA2000 network to allow voice calling and SMS text messaging.

I say "supposedly" because Verizon is making absolutely no promises that this phone will work. This is all coming from HTC. The phone hasn't been approved by Verizon. But HTC says it will work.

CDMA has been an issue for some phone makers because CDMA voice calling is not used much anymore outside Verizon, Sprint, and China Telecom, and Verizon and Sprint users don't buy a lot of unlocked phones. (There are other CDMA carriers, but they're generally either small, data-only, or have plans to switch to GSM.) As testing and verifying additional radios is expensive and time-consuming, that's caused most unlocked phone vendors to shy away from CDMA.

(For more on the difference between CDMA and GSM, see our GSM/CDMA explainer.)

Until now, only Apple, Google, and Motorola have provided unlocked phones compatible with Verizon, by including CDMA radios. Allowing LTE-only phones makes it easier and cheaper for phone makers to offer Verizon-compatible devices.

The One A9 will run voice calls, SMS, and MMS exclusively through LTE on Verizon's network, HTC says. Verizon declined to confirm this. If it works, it will have the knock-on effect of enabling high-definition voice calling to other Verizon phones. Verizon says its LTE coverage is now essentially the same as its 2G coverage. The main downside is that Verizon users will lose coverage anywhere they would have dropped to 2G only, such as deep within some buildings or underground.

A lot of questions remain here. HTC and Verizon both say that Verizon won't activate the A9 with a new SIM or plan; you have to use an existing, active Verizon SIM and slot it into the A9. Verizon stores also won't be able to activate the phone. That may be because Verizon's activation system still relies on CDMA.

Related Hands On With the HTC One A9

Beyond the A9, making voice calls may require Verizon-approved software. So you may not be able to bring just any LTE device to Verizon and have it work as smoothly as GSM devices do on AT&T and T-Mobile. I'm not sure yet. Verizon hasn't gotten back to me.

There are a bunch of barriers to doing this on Sprint, by the way. Most notably, Sprint doesn't have anywhere near the LTE coverage Verizon does, making 2G CDMA support much more important. Sprint's LTE system is also more complex than Verizon's, combining two different forms of LTE, so it's less common in global device versions.

The $399 HTC One A9 goes on sale today via htc.com in two models: the AT&T/T-Mobile/Verizon version, and a special unit for Sprint.

Further Reading