Use a phone?   Any type of phone?   Better understand SS7

The most important takeaway is to understand that the telephone network that your phone connects to - every phone on planet Earth and every call made - is not secure and probably never will be.

There may be some improvements in the future with regard to telcos scanning for unauthorized users, but that won't stop a determined player. Telephone networks were not designed to be secure. Understanding that and adjusting habits may help when security is needed
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What can I do to protect myself?  Nothing really...

Given that the vulnerabilities and the possibilities of spying on users relies on systems outside of user control, there is very little you can do to protect yourself beyond not using the services.
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