When you release a number, Twilio will reserve that number for your project for 10 days in case you need it back. After 10 days, the number will be transitioned to the next stage of its lifecycle, where it undergoes a series of traffic and capability testing. This is to ensure that phone numbers are clean with minimal traffic before they are made publicly available.
By using Console, you can restore numbers that you released in the past 10 days. Release numbers will show up on Console's Released Numbers page. Simply click on the Repurchase Number button to provision the number back to your project.
Note that the action is considered to be repurchasing the phone number, so you'll be charged for the monthly fee upon performing the action. In addition, the restored phone number will be given a new Phone Number Sid. Make sure you update the record in your application before accessing the phone number resource.