Before we proceed, I have some advice for you. As soon as we update the mobile number. You'll receive a free text message stating that your previous mobile number will be unenrolled from the services. As soon as you receive such free text messages, please restart your devices. But no worries though, the new mobile number will carry all the services that you have.
Your voicemail box and old messages will also be deleted. You could actually back it up first if you want to keep them. If you have a CallerTunes, you might lose the retired CallerTunes features and it cannot be resented nor refunded. Regarding your T-Mobile account, you don't need to re-register. You can just use the new mobile number and provide the existing password and you also need to clear the application cached and data.
As soon as we update the mobile number, please wait within 2-4 hours to complete the changes to your account. Should we proceed right now?
Were you able to back up all the necessary contact list and voicemail as well?
One more thing, XXXXXX. Just update also your any account where you registered your old phone number such as bank accounts, social media, account verification, and notifications.