Sounds like the old phone still has your number in the settings.
It is under settings > my phone > my number.
Change this, hit save and reboot phone.
Should fix the issue.
You might also want to check whether your Apple id is logged in on the old phone too...
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
