The short answer
Secure the carrier account that controls your phone number before an attacker can move it to another SIM or eSIM.
Step by step
Use a unique carrier-account password and a carrier account PIN that is not reused for voicemail, banking, or email.
Turn on the carrier's number lock, port lock, or transfer protection for every line when that option is available.
Protect the email account tied to the carrier with an authenticator app or security key instead of relying only on text messages.
Remove outdated authorized users and confirm that account-recovery phone numbers and email addresses belong to you.
If service suddenly disappears, contact the carrier from another phone, lock financial accounts, and change important passwords immediately.
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.