The short answer
Prepare account access and a recovery path before moving the digital SIM that controls your number.
Step by step
Update and back up both phones, and verify that the new phone is unlocked and compatible with the carrier.
Keep the old phone connected, charged, and signed in until the new line is fully working.
Use the carrier's supported device-transfer flow or request a fresh eSIM through the official app or account.
Test calls, texts, data, voicemail, hotspot, and authentication messages on the new phone.
Only then erase the old phone or remove its eSIM, and save any activation confirmation.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.