It happens to me all the time. It’s called “neighbor spoofing.”
The number that shows up on your phone for Caller ID is not generated by the phone company, it’s generated by the phone that’s calling you. It can be spoofed.
A recent trend in spam has been to call your phone and “spoof” your caller ID display with a local number, on the theory that you’ll be more likely to answer a call from an unknown local caller than from a caller in another area code.
It appears that a recent update to the Apple iPhone software
sends out a text message when you send a call to voice mail, in addition to recording audio from the caller. That text message goes to the Caller ID number in your phone, so the person who’s number was used (the “spoofee”) gets a message from you, that you didn’t actually type in.
As I said, it happens to me all the time, sometimes more than once per day. It’s very annoying. I’ve tried to turn off the text-message feature but I haven’t been able to do it.
My iPhone 6 with IOS 12.0.1 doesn’t do that, fortunately. I have not upgraded yet to 12.1. I do use Malwarebytes which filters some of the suspected spoofed calls (not perfectly).