Setting Up - A Few Questions

I have my fi SIM card and I'm ready to make the switch.

I'm bringing my current iPhone 7 from my old carrier (USA - Credo).

I've finished my 2 years and own the phone.

- Do I call my old carrier to cancel my service and unlock the phone and then, drop in the new Fi SIM card?

- Is there a specific order to canceling, unlocking, switching to fi to ensure I retain my historic cell phone number?

Thank you