Which smartphones block call recording? Every brandname Android I have had enables this feature for voice calls. (Although the feature is missing for app calls, and I'm now at least 5 years behind the newest phones.)
This explains a lot because this is a more recent development. https://www.pcmag.com/news/google-is-banning-call-recording-...
As the article says it was blocked a couple years ago but there was still a loophole which is now being closed as well. Though it's by blocking apps using it from the play store which can still be avoided by sideloading so technically there still is a loophole :) But it's definitely seeing more and more restrictions.
Except at Google and Samsung apparently. And there's no way to contest this decision for users, query it or even get a straight answer out of either company on it.
iOS?
I'm not sure of the current state of Android but back in the early days this feature also wasn't there (there were tons of apps that claim to do it but they all exploited some device-specific quirk and wouldn't work universally) despite the hardware being capable of doing this from day 1.
The reason call recording is shunned is that tons of scummy companies would suddenly end up in trouble if call recording becomes widespread and accessible to anyone.
(And in any case, as you suggest most people pay more per bit transmitted on a mobile voice call than they pay per bit as data.)
Purely for fun, it would be interesting to hack up two smart phones to transmit data over a voice call. Presumably, you can keep everything binary, if you control the end-points.
Practically, you‘d be better off using a throttled (after exceeding the included data volume) native data connection.