What you said is correct. What I was referring to is the fact that you cannot be active on a call and receive a call from the other phone number. Instead it goes straight to voicemail.

Per https://support.apple.com/en-us/HT209044 :
”When you're on a call, an incoming call on your other phone number will go to voicemail, if you have voicemail set up with your carrier.* However, you won't get a missed-call notification from your secondary number. Call Waiting works for incoming calls on the same phone number. “