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How much data does a Voice-over-LTE call use, and does it bill against your data plan?

Yesterday a friend of mine sent me this text message:

So, I spoke to two separate AT&T representatives, one on Tuesday and the other yesterday, and they both said that VoLTE calls when available will subtract from monthly cellular data.

So, I spoke to two separate AT&T representatives, one on Tuesday and the other yesterday, and they both said that VoLTE calls when available will subtract from monthly cellular data.

Well, I can’t well ignore that. I reached out to AT&T via Twitter for comment:

@ATT @ATTCares Can you comment on VoLTE data usage?

@ATT @ATTCares Can you comment on VoLTE data usage?

Not receiving a response, I decided to do some ballparking on my own. I wanted a rough estimate of the peak usage that VoLTE could use under any circumstances, because, what if it’s true?

So, first thing to note is that under AT&Ts roll-out of VoLTE, they’re introducing HD Voice. HD Voice is also being rolled out on other carriers, some possibly independent of a VoLTE roll-out, but since I’m focusing on bandwidth utilization, that difference is not applicable to this.

Now, for the purposes of VoLTE, there are two codecs in play here. AMR-NB (for narrowband, normal calls), and AMR-WB (for wideband, HD Voice calls) [source]. AMR-WB will naturally have the higher bandwidth utilization of the two.

The AMR-WB codec uses, at peak, a 23.85kbps (kilobits) codec [source]. That’s one-way, making it 47.7kbps peak for both ways. I’ll say 48kbps to add a little overhead. This number is in line with an article at NetworkWorld, which states that VoLTE calls come out to about 30-40kbps at peak. That comes out to 6kBps (kilobytes), both ways, at peak. Roughly speaking. It’s possible for the codec to shift to a lower bandwidth during silences and lower frequency-range conversations, or during network congestion, and that’s important to understand.

So, doing the math on a 15-minute phone call will give you a peak usage of 6kB/sec, 360kB/min, and 5.4MB/15min (1MB=1000kB). That’s absolute peak. All other usage not withstanding, that’s [very] roughly at least 92.5 hours or 5,550 minutes of talking on your 2GB plan.

And that’s if VoLTE calls bill against your data plan.

UPDATE: A reader has sent me the following from Verizon’s website, which specifically mentions HD Voice, but not VoLTE:

HD Voice is available at no additional charge and is included in existing plans.
HD Voice calls are billed as standard voice calls according to your plan. No data charges apply.

Mobile-to-Mobile calls that happen to be HD Voice calls are charged just like traditional Mobile-to-Mobile calls and are billed against your monthly minute allowance according to your plan.

A video call is an HD Voice call combined with real-time video. The voice portion is billed as a standard voice call, according to your plan. The video portion is billed as data, according to your data plan. No data charges apply to video calls transmitted over Wi-Fi.
Note: An average 1-minute video call uses about 6 – 8 MB of data. The actual data consumption of your video call may vary. You can estimate your data usage using our online Data Usage Calculator.

Source: https://www.verizonwireless.com/support/advanced-calling-faqs/

Thoughts on this? Corrections on my math? Please feel free to comment below. 

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