Thanks for your responses!
I am thinking about to take the service from a person who offer phone verification. But isn't is a security problem, is it? When I use a other persons phone number for verification then this person could use the phone number to reset the password and occupy the account, right? Or is it possible to choose that the phone can not be used to reset the password?
Nah, the mobile number does not have that power to reset the password. It's just for verification purpose. As long as you have access with your email, and if you are the only person who knows your twitter password then your account is safe.