New iPhone

I just purchased an iPhone 12 Pro Max from Apple. I asked them to notify my carrier when I bought the phone and just set it up. That went smoothly.

Now, my question is what do I do with my old SIM card, and my old phone? I see that non of my voice messages transferred to my new phone. Is that because they are on my old SIM card? Is there a way to save them on my new phone, or do I have to put the old one in to listen to those messages?

Before I can sell it I will need to reset it and wipe it. Can I do this even though my phone number no longer is associated with that phone? Will my phone carrier charge me for a 2nd line if I use it to reset, or is that ability built into the phone device itself, separate from an actual phone line?

I don't recall having these questions the last time I bought a new phone, but that was going from a 4 to a 7---a long time ago.