That's true, although those of us with unlocked phones can purchase "throwaway" SIM cards with distinct phone-numbers fairly easily with cash, use the number for secure communications only, throw the card away, and then go back to our public lives.
You gotta burn the phone.
So other than full-on burner smartphones (ie: throw away $600 every time you need to do something privately), what's a reasonable heuristic for conducting private business? Maybe collectively owned or rented burner phones so that the metadata collector only ever knows, "Someone rented this device for encrypted communications" rather than who in specific is communicating with whom?
You need to do a threat analysis.
I did not immediately find any good introductory resources via a quick Google search, but try it yourself. Very very briefly, it involves identifying the threats and their possible consequences, then working on either removing the former or minimising the latter.
Be aware that this is not merely a technological process. It is primarily a social one.
You can set up an SMS gateway number just for this purpose, but you shouldn't need such workarounds.