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Comment by ebb_earl_co | Hacker News Reader

From [0], it seems that RCS can use the cellular signal instead of LTE or 5G:

> In cases where RCS is able to operate over cellular networks without data, it supports messaging as well as file transfer, enriched calling, and more.

[0]: https://en.wikipedia.org/wiki/Rich_Communication_Services

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In LTE and 5G there exists no cellular signal without data. That was a 2G/3G thing. Since 4G everything is data. The separate sms and voice services are gone. And 3G is rapidly being deprecated.

That's not quite true. VoLTE was an optional part of 4G in the beginning. I had one of the gaming ASUS phones that shipped with 4G, but no VoLTE. 6 months after getting it, the US phone carriers conspired to make VoLTE mandatory for connecting to their towers, effectively bricking my device. ASUS never issued the firmware update that would have enabled the feature in the modem, which had the capability, and the community hack to enable it never worked for me.

Yes, circuit switched fallback was possible for a time (LTE data and circuit-switched 2G or 3G voice). I think 3GPP shouldn't have written that into the spec in the first place, but you know what they say about hindsight....

The whole saga with VoLTE was really quite entertaining: https://nickvsnetworking.com/background-to-the-volte-mess/

4G without VoLTE is using 3G for voice because there is no other 4G voice.

It was an optional part but without VoLTE you just could not call with 4G. For that the phone just switched back to 2G/3G.

That's not a sustainable solution because 2G/3G are getting switched off. And it doesn't really have any bearing on LTe itself.

The real issue is that they didn't really standardise VoLTE enough. Leaving every carrier and manufacturer to have a mesh of different options and not every carrier and every phone supports the exact same subset of options, which causes the compatibility issue. Also sometimes providers don't bother checking phones they don't sell.

They really should have standardised it 100% so this couldn't happen.

It's true that it's all packet-switched rather than circuit-switched, but IMS traffic is still handled differently than regular data traffic. (The most user-visible implication of this is the QCI difference: cellular phone calls are much more reliable at poor signal strength and high congestion than over-the-top VoIP calls).

Isn't that just basic QoS?

It's a totally parallel IP stack that's handled differently on the device and the base station hardware. Being on a call will affect what frequency bands your phone uses, handoff behavior, carrier aggregation, and other things that over the top calls like WhatsApp audio wouldn't.

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