I think 2FA is to guard against someone gaming their "lost my password" process as much as against someone brute-forcing your pw.
Have you checked: https://twitter.com/settings/apps_and_sessions
It is not unreasonable to assume that this has been changed to the attackers number.
Then, the hacker just gets a local number, which should be easy enough these days.
EDIT: Ah I see elsewhere you said you don't use a smart phone (thus, no authentication app.)
Regardless that still wouldn't protect you against somebody social engineering their support team. The account security page at https://twitter.com/settings/security has an "Additional password protection" but that seems to inhibit automated password reset requests, not support-related ones.