>how can you know that and avoid texting this new person sensitive information
You can’t. Number portability means a person doesn’t even have to die for a number to be reassigned to another person.
To illustrate, I’ll use my own personal example. My wife and I have paid for a cell phone for her mother (my mother-in-law) for a number of years now. Some of her doctors have that number as a contact number. She’s in her 90’s and is at the point where the cell phone is not needed. We’re considering giving up the number. When we do, that number will be reassigned to someone else. And yet, my mother-in-law will still be very much alive and even visiting some of those doctors.
It’s on the sender of the messages to ensure that the number they are sending the messages to is actually tied to the person they intend to receive those messages. Doctors often get around this by sending something along the lines of “Your latest results are available on your patient portal. Please call our office at [xxx-xxx-xxxx] if you have questions.”
If this is person to person communication, then perhaps messages like “This is OP. Give me a call when you get a chance. Thanks.” If the number has been reassigned, either you’ll get no answer or they’ll text back “new phone, who dis?”. Until you get confirmation though, you know to be careful about what you send.
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