If you don't recognize the trusted phone number on your iPad, first make sure that your account is Safe by following the instructions in this link If you think your Apple ID has been compromised – Apple Support (UK)
After that, you can follow these instructions to remove the telephone number
Update your trusted phone number on your Apple ID account page
- Go to appleid.apple.com.
- Sign in with your Apple ID.
- Go to the Security section and click Edit.
If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click next to the phone number you want to remove.