Before the old phone leaves
Read eligibility, condition requirements and the deadline. Record the device you are sending and its condition without exposing private information. Follow the provider's instructions for backup, account removal and erasure after the new device is ready.
Confirm which accessories or packaging are required. Do not send extra equipment simply because it arrived with the new phone. If an installment or credit condition is unclear, obtain the terms before shipping.
Delivery is not the same as acceptance
Keep the shipping receipt and tracking confirmation. A delivered parcel can still await inspection. Check for the provider's acknowledgement and final trade-in assessment, not just the courier's delivery event.
If a credit is applied over bills, note the expected start and duration. A delayed first credit and a rejected trade-in are different issues. Use the official case process with the relevant evidence rather than resubmitting unrelated documents.
Before another account change
Check what a plan change, early payoff, upgrade or line cancellation does to future credits. The old phone's acceptance does not prove all future credits are unconditional. Keep records until the credit arrangement and any device balance are fully reconciled.