trying to create a new developer tenant, but it is saying I can't because I had one under my former company using my personal phone

To create a new Microsoft 365 Developer Program account, you must first delete your existing account associated with your personal phone number. You can do this by signing in to your dashboard and selecting Delete Profile. After you delete your existing account, you will need to wait for more than 60 days before you can create a new subscription.

If you are trying to disassociate your personal phone from the old developer account, the best approach is to delete that account as mentioned above. Once the account is deleted, you can then proceed to create a new developer tenant with a different email account or account type.

References:

• Microsoft 365 Developer Program FAQ
• Microsoft 365 developer subscription in Visual Studio subscriptions