Mobile telephony support just comes as a module, so that’s actually the easy part.
The harder parts are to make the whole thing consume a low enough amount of power that you can keep it running from a non-monster-sized battery, and I suspect that an RPi 5 board isn’t very good for that (hobbyist development boards tend to not have been designed to avoid wasting power, even when the underlying microcontroller/processors is actually decent at it), and integrating an OS with support for a touch interface, especially if you want to avoid Android.
I mean, it’s not too hard to make a brick sized dumb phone and even have it be a mobile phone powered by AA batteries, but if you want a mobile smartphone, it gets more complex.
Unless you have the time and skills to take up the challenge you would probably be better of getting something like a Volla phone with Ubuntu Touch or a Pine phone.
What if you take the battery from a small laptop? It may still be bigger than a whole smartphone, but comparatively small and with some ten watt-hours.
Even a smallish LiPo battery will give you some time. The question is how little becomes too little.
And on the other size you can always give it more battery: after all the original mobile phones were the size of a briefcase.
Ultimatelly how bad it is to go with a Raspberry Pi depends on how much more it with the software it has consumes than what a custom circuit designed for saving power using software configured for that (for example, not running needless services). Further, how much would, say, the extra power used in an HDMI connection over other more lower level protocols of talking to a display really matter next to the power consumption of the display itself or the GSM module, both of which tend to be big power users?
I know for sure that if you design a custom board with a basic STM32 microprocessor and add a 2G GSM module to it, most of the consumption ends up being the 2G module anyway, so you could probably get away with just using some hobbyist board with it instead of designing your own with just what you need and a proper Voltage Converter. However I haven’t really tried doing a battery powered smartphone with an ARM SBC so I don’t really know for sure.
Mobile telephony support just comes as a module, so that’s actually the easy part.
The harder parts are to make the whole thing consume a low enough amount of power that you can keep it running from a non-monster-sized battery, and I suspect that an RPi 5 board isn’t very good for that (hobbyist development boards tend to not have been designed to avoid wasting power, even when the underlying microcontroller/processors is actually decent at it), and integrating an OS with support for a touch interface, especially if you want to avoid Android.
I mean, it’s not too hard to make a brick sized dumb phone and even have it be a mobile phone powered by AA batteries, but if you want a mobile smartphone, it gets more complex.
Unless you have the time and skills to take up the challenge you would probably be better of getting something like a Volla phone with Ubuntu Touch or a Pine phone.
What if you take the battery from a small laptop? It may still be bigger than a whole smartphone, but comparatively small and with some ten watt-hours.
Even a smallish LiPo battery will give you some time. The question is how little becomes too little.
And on the other size you can always give it more battery: after all the original mobile phones were the size of a briefcase.
Ultimatelly how bad it is to go with a Raspberry Pi depends on how much more it with the software it has consumes than what a custom circuit designed for saving power using software configured for that (for example, not running needless services). Further, how much would, say, the extra power used in an HDMI connection over other more lower level protocols of talking to a display really matter next to the power consumption of the display itself or the GSM module, both of which tend to be big power users?
I know for sure that if you design a custom board with a basic STM32 microprocessor and add a 2G GSM module to it, most of the consumption ends up being the 2G module anyway, so you could probably get away with just using some hobbyist board with it instead of designing your own with just what you need and a proper Voltage Converter. However I haven’t really tried doing a battery powered smartphone with an ARM SBC so I don’t really know for sure.