To improve security, Twitter has introduced two-step authentication for its users.
The new two-step authentication system requires you to know your username and password and have your cell phone with you as Twitter will send you a code via SMS that would be required to log in.
If your cell phone is missing, dead or out of range or there is any other issue due to which you cannot use your cell phone, there is an alternative to use this service/ It’s Google Voice.
Instead of registering your actual mobile number with Twitter, you can register a Google Voice number. Google Voice is free and easy to set up and lets you forward text (SMS) messages and phone calls to several phones so that an incoming text or call can be answered or viewed from any of the phones associated with your Google Voice account. So now you can only get the SMS on your mobile via Google Voice but also can view it on the web when required.