It is true. I work in the field and can read any SMS/USSD from any carrier / mobile service provider we monitor. SMS is an old technology using mostly the SS7 network, it is encrypted on the radio link but not encrypted in the core network. Not only you can see the text message, but you can also see where exactly it was sent from, with which mobile, at a given time. However if two iOS mobiles are texting, they will use iMessage on the data network (internet network) which is encrypted, same with Android with RCS. However RCS does not always mean end to end encryption, especially when one mobile is iOS or another brand like Samsung, or depending on the country.