Why does my new iPhone show received and missed calls from my old phone that has a completely different line?

This will happen when both of your iPhones are signed in to the same Apple ID. Both iPhones are probably sharing the contacts, Texts, Notes etc. as well. If you would like to keep all data separate between the two iPhones then you will have to keep them signed into separate Apple IDs.

Axel F.

No worries. Calls made to your T-Mobile phone will remain on T-Mobile bill even though the other iPhone may ring and even if you answer on the other iPhone. And the same holds true for your pay as go iPhone.

Axel F.

No. Calls made directly to each iPhone will remain and show up on their respective statements. You won't see the other iPhone's calls on your bill and vice versa.

Axel F.