@nprofile1q... So basically, normally you can only connect to another SIP user via the Internet as an IP interaction.
A trunk was originally to do with how phone systems worked and actually physically linked you into the network.
SIP trunks are a virtual equivalent of this that mean your SIP PBX server (which was once upon a time a physical machine or many for telephony interconnections) can now communicate with the normal phone system.
So what I'm saying is either the government or random private operators should provide SIP accounts for customers/whatever (one should have the option of self-hosting, if one is sufficiently masochistic), at which point all one needs to make a call is a SIP client. Which can be mobile, desktop, or even embedded in IP phone hardware.
Making the cellular network a special snowflake thing that requires special (untrustable) hardware isn't necessary and hasn't been for a very long time (if it ever was).
But instead of this more sensible take on telephony and telecommunications, SIP trunking is basically exclusively the domain of expensive business shit you need to do yourself (or pay a specialist company to do/setup for you).