SNS - Cannot Delete origination Phone Number

Removing numbers depends on the type of number you added. You didn't specify in your question, so I'll provide both options below.

To remove dedicated short codes, you will submit a case in AWS Support Center. You can find the instructions for filling out the support case in the Knowledge Center post How do I delete the dedicated phone numbers that I purchased through Amazon Pinpoint....

For dedicated long codes, toll-free numbers, or 10DLC numbers or campaigns, or company registrations, you will want to navigate directly to the Amazon Pinpoint console, and in the left navigation pane, under Settings, choose SMS and voice. From there, follow the instructions in the Knowledge Center post How do I delete the dedicated phone numbers that I purchased through Amazon Pinpoint....

If your numbers are located in the sandbox, you will need to wait 24 hours after the last time you verified before you can delete. Once that time has passed, you can try again.

Thank you.