I mean how do you start as new brand then ?
Bribe them ?
You can't buy chips. You would need to prove them, that you can pass GSMA FCC CE and other certifications before they would even consider selling anything to you. All this is like 100KEUR budget, if everything passes first try.
Buy a module. Most modules have either direct audio in/out or I2S. Putting together a phone to call is not that hard for an experienced engineer. For example page 157 below shows you how to make a phone.
https://www.u-blox.com/en/docs/UBX-13004618But then you have a board that does nothing. Then you need to start antenna matching from almost DC to daylight. That can take a few days getting it right.
And then writing software. Which can be ridiculously complicated. The issue is not making a call, that is a few AT commands. Where the complexity comes from is network providers doing all kinds of crazy stuff. They do this for political reasons, backwards compatibility, network limitations, and so on. You want to roam in France, and you suddenly cannot connect to your preferred network. And the modem only tells you that you have some sort of unspecified error. You search the error, and there is like 1 forum post from 2011 from a guy who tried to do the same as you. Nothing is documented about these things. And this is just the tip of the iceberg.