How is my Mac able to make phone calls when my iPhone is off?

I just switched from an Intel Mac to an Apple Silicon M2 Mac.

With my old Mac, I could place phone calls using FaceTime, and the call would go through my iPhone. I could continue using the Mac mic & speaker, or pick up the iPhone and switch to the phone.

With my M2 Mac, the option to switch is gone. My iPhone acts like there's no call in progress.

What's really weird is that I discovered that I can place calls from my Mac even if I turn my iPhone off completely! (The person on the receiving end sees my cell number, so it looks as if the call has originated from my phone.)

What's going on here?

I use T-Mobile, if it matters. But I haven't installed any T-Mobile apps or other software to route my calls directly over the internet.