Why do I still receive phone calls when cellular is off

I'm aware of no option that will turn off cellular phone calls other than Airplane mode or completely turning off the phone.

How do you think that you did that -- i.e., what options did you turn off?

Yes, so a lot of carriers support Wi-Fi calling, which improves call quality [on Wi-Fi], and enables you to make and receive calls on poor - no signal…

Make calls using Wi-Fi on iPhone – Apple Support (UK)

Settings > Cellular > Cellular Data only turns off data, not calls.

Try turning Settings > Airplane Mode on: Use Airplane Mode on your iPhone, iPad, iPod touch, and Apple Watch - Apple Support and then turn Settings > Wi-Fi back on.

Or if you just don't want the phone to ring, use the ring/silent switch. How to put your iPhone on vibrate, ring, or silent mode - Apple Support

I'll assume these are voice calls to your iPhone.

Is there another path for Wi-Fi calling and arriving telephone calls here? Second iPhone, some other device with Wi-Fi calling available, or maybe a second telephone number associated with your same iPhone?

Make and receive calls on your Mac, iPad, or iPod touch - Apple Support

Settings > Cellular > Cellular Data only turns off data, not calls.