Twitter – How to change phone number on Twitter

If you can't see it anymore from the Twitter Web interface, they have been deleted for good.

If by mobile you mean an application, then they're being cached. Your mobile app doesn't know that you deleted them or doesn't have feature to remove them from cache once they have been deleted. If it really bothers you, Clear cache and/or data of your app, and they should go away.

Twitter's help pages have some info on this:

• My email, phone number, or username is already in use
• How is my phone number associated with another Twitter account?

but their only real answer for your question is this part:

• Still need help? Contact Support.

I suggest you do that.

Alternatively, ask your phone service provider to change your new phone number and hopefully refund the minutes used by the number's previous customer. Or maybe they can block Twitter for you.