The question, I would ask, is, are there any employees who are called by nicknames that are not their legal name. I.e. anyone named Robert called "Bob", anyone named "William" called "Bill", etc.
If yes, then their refusal to call you by your preferred name is discrimination. I.e. they are doing it because Ben is a male name. If you had a female nickname they would have no problem with it.
If they persist, I'd say your next step would be a local LGBT legal help place or maybe the EEOC.