For purely data, sure, that works. I have actually done it for multiple projects but for industrial purposes. It is slower than what a phone can achieve with an integrated baseband and SoC but would be good enough
For VoLTE, it is possible to get very basics with external modules but it is also very time and operator dependent. You need to have the profiles of each and every single operator you may support. If the phone would be globally available, this means thousands of profiles. Your module needs to be configurable after deployment. You still carry the risk of operator changing their profile or switching to a different encoding that your module doesn't support.
RCS is completely proprietary to the specific operator. There are currently no external modules that supports it, nor I beleive will be due to the complete proprietary nature. Google and Apple internally handle their pairing with the ones that support it.
On top of these two, you have actually a significantly bigger problem. The reason companies like Qualcomm integrate baseband chips with the main SoC die is the power efficiency. With external modules you will never reach an integrated circuit efficiency. Moreover you are sacrificing valuable battery space to the extenal module.
This is all true, and my phone (from the link) is not very energy efficient and doesn't support all mobile bands. But it exists and is usable.
Calls work for me, too.
> You still carry the risk of operator changing their profile or switching to a different encoding that your module doesn't support.
