All points well taken but I would suggest there is one way to have high assurance in this regard ... and that is to buy a non-phone device and add a cellular USB dongle to it, after the fact.
Pretty unwieldy but given the economics of secretly throwing in a full-blown baseband processor with a SIM card that can function I think you can have high assurance that your carrier is fairly well segregated from your device ...
In fact, there used to be a particular Samsung galaxy "pad" or something that was identical to a phones form factor ... but wifi only. EDIT: Samsung galaxy "player" IIRC ?
As long as we're talking about it, I will mention one oft-overlooked downside to running a phone via external modem: in addition to cellular network functions your baseband processor is also responsible for a lot of real-time voice quality corrections and echo cancelling and related functions that are not handled by the application processor. You might, therefore, have voice quality issues with such a setup ...
> In fact, there used to be a particular Samsung galaxy "pad" or something that was identical to a phones form factor ... but wifi only. EDIT: Samsung galaxy "player" IIRC ?
Yup, and on the Apple side there's the iPod Touch (which is apparently still available!?)
Oh, you meant a literal modem-with-a-USB-port... okay, yeah.
An alternative might be to use one of those standalone cellular-to-wifi hotspots, connect the iPod touch to it via wifi, and then use a VoIP app. That way you can keep the hotspot in a backpack, perhaps with a big power bank / battery, rather than dealing with the unwieldy physical dongle.
Pretty unwieldy but given the economics of secretly throwing in a full-blown baseband processor with a SIM card that can function I think you can have high assurance that your carrier is fairly well segregated from your device ...
In fact, there used to be a particular Samsung galaxy "pad" or something that was identical to a phones form factor ... but wifi only. EDIT: Samsung galaxy "player" IIRC ?
As long as we're talking about it, I will mention one oft-overlooked downside to running a phone via external modem: in addition to cellular network functions your baseband processor is also responsible for a lot of real-time voice quality corrections and echo cancelling and related functions that are not handled by the application processor. You might, therefore, have voice quality issues with such a setup ...