You can have LTE voice even if you've maxed your data cap, from a technical point of view. For 2G/3G it's true that those standards support pretty independent CS (voice) and PS (data) connection, so it's natural to decouple voice and data. For LTE, and also NR, only the PS domain exists. It's still possible to separate Internet access from voice though, and they are. It's just done in a different way. 2G/3G/4G support multiple concurrent data connections, called PDN. Each is like a different IP interface, and some are terminated on the modem itself and not exposed to the user. In the typical LTE deployment, you have an IMS PDN for voice signaling and data, an admin PDN for the remote management of the telco using OMA-DM, and an Internet access PDN for the end user --- and the only one you'll see. Traffic on those PDN are fully segregated, and counted separately. So the IMS and Admin PDN traffic do NOT count as part of your data cap, it's on top.
Thanks to this LTE IMS voice is separated from data, like 3G even if it's done differently.
Of course, this doesn't apply to OTT voice: that is always in the Internet PDN and in your data cap.
To get back to 3G, it'll slowly die. Anything you can do on 3G you can do better on 4G. So operators tend to use their 3G infra to the max, but they won't upgrade it. It'll be replaced by 4G over time, until they eventually pull the 3G plug.
Thanks for the technical reply but I meant for the situations where the network coverage is poor and you don't have a good enough 4G signal to place the call. I would fully expect the provider to not include LTE voice in my data allowance, in the same way they often exclude some streaming/social network services.
Ok, thanks for the clarification. Even with this new understanding 3G will eventually go. Coverage depends on several things: 1) deployed infrastructure, 2) frequency used and 3) waveform / standard efficiency.
3G can have a temporary advantage for (1). But as the 3G infra ages, it will always tend to be replaced by cheaper / more efficient LTE infra as the existing 3G bands can always be refarmed for LTE. And LTE will make a better use of the available bandwidth. There are large differences between operators on the speed of this process, but the trend is universal.
(2) is why 3G was impaired vs 2G: 2G was deployed in low bands (900 MHz for 2G is very common), while 3G was very often only available in mid-bands (1700+ MHz). The higher the frequency, the more challenging the coverage. So 3G coverage lagged 2G for a long time, and is still lagging in places (Europe for example).
The situation is different with LTE, due to the digital TV dividend. This opened low bands for LTE everywhere (700 to 800 MHz, depending on region). Those bands are even better then the 2G bands. They're not yet fully deployed everywhere (Europe is lagging), but they eventually will. And when it's done you can kill 2G and 3G.
For (3) or course, new tech has improvements so 4G is better than 3G and 2G.
When you combine all of this LTE will replace all previous technologies everywhere, in time. How long it will take will however vary a lot depending on regions. Europe invested a lot in 3G, and telco there will milk this infra as long as possible. And because 3G coverage in Europe is not as good as 2G (see above, bands) 2G will also tend to stay with 3G. Cheap second hand 2G/3G infra will remain for a long time in developing countries. But in the end 2G and 3G will go away, it's just a question of time.
On the other hand, 5G will take a lot longer to replace 4G. The big change is mmWave, but it's for small hotspots. You can have 5G in low bands too, but then the gain vs 4G is small so I expect operators to take their time to replace their 4G infra. We'll have 4G for coverage + 5G for dense areas only for a long time IMHO.
If they kill 3G, they'll reuse those radio bands for LTE or 5G, and you'll get the same coverage you have on 3G now.
Around here, LTE coverage absolutely kills 3G coverage since LTE is deployed on the old analog TV bands (700/800 MHz) which has great penetration into buildings etc. My operator doesn't even activate 3G on new iPhones - they're effectively LTE-only now.
Thanks to this LTE IMS voice is separated from data, like 3G even if it's done differently.
Of course, this doesn't apply to OTT voice: that is always in the Internet PDN and in your data cap.
To get back to 3G, it'll slowly die. Anything you can do on 3G you can do better on 4G. So operators tend to use their 3G infra to the max, but they won't upgrade it. It'll be replaced by 4G over time, until they eventually pull the 3G plug.