I don't know if this is still the case, but in the past when I changed a line to a different SIM on T-Mobile an alert about the SIM change was texted to me, on the new SIM. :/
It didn't inspire much confidence in me regarding their security practices
Once there was a billing snafu and they cut off my line, with no notice given. I was freaking out a bit cause I thought it might be a SIM swap attack or something. After figuring it out and getting reconnected, I realized that actually they had told me about it. Via a text message. To the phone number they disconnected. After they disconnected it.
It didn't inspire much confidence in me regarding their security practices