Hello,
I hope you're doing well.
There’s no self-service way to delete or reset a Microsoft 365 Developer account, and the Dev Program team can’t remove a phone number from it. The only way to free up your number is to contact Microsoft Account Support directly: https://support.microsoft.com/en-us/home/contact?SourceApp=smc2&ContactUsExperienceEntryPointAssetId=account.microsoft.com.
Just tell them you picked the wrong setup (Personal instead of Business) and now your phone number is stuck on that old tenant. They can unlink it for you so you can start fresh with a new Developer Program account.
Best regards.