Creating a consistent brand across all social networks is important and that is why it can be very stressful if you go to set up your Twitter account and find out that your desired Twitter Handle isn’t available and is in use by someone else.
Social Media Today published a great post with some tips on how to deal with things when you can’t get your Twitter Handle . ( What to Do When Your Twitter Handle is Taken But Isn’t in Use ).
Here are some key links for you to deal with a situation when you are unable to get your Twitter Handle and you really are entitled to it.
Your Twitter Handle is not available and has the big fat egg or hasn’t been updated in several months. Refer to Twitter’s Inactive User Policy
Your Twitter Handle has been taken over by someone else. This is typically used when potential trademark issues are involved. Refer to Twitter’s Trademark policy violation
Someone is actually pretending to be you on Twitter. You will want to reference the impersonation help team at Twitter