By his late twenties, E. B. White was a well-paid writer for the new and rising star weekly, The New Yorker. How did he get there?
Calling certainly starts with who you are, but it's found through what you do.
White edited the college newspaper at Cornell, but after graduation he found work in the advertising business, as a poorly paid and often part-time copy writer. He viewed the job as a means to an end: he continually submitted short paragraphs, poems, and other small pieces to the popular compilation columnists of his day. After he published a few short stories, The New Yorker's publisher sought him out. He started as part-time writer but soon became a key author for the weekly magazine.
He knew he was supposed to be a writer and kept at it, even when his best efforts resulted in uncredited (and unpaid) bits in other people's columns. That's how you find your calling. Do it!
White's calling was not an overtly predestined route. He found calling in hundreds of small and innate decisions.
Want to be a political writer? Write political stuff and submit it. Want to do something in research? Do and publish your research. The barriers to publication are almost non-existent. Apple released a publisher that lets you create and sell books on the iBook store for free. You will find your calling in concrete steps.
And also this: while he was an influential columnist and essayist, it was his downeast Maine farm that gave him inspiration for Charlotte's Web. Summering in and later moving to Maine made his work culturally meaningful. That's cool. Maine is cool.