Today I got a phone call from a consulting company and they told me that I can only file H1B after I start working and get W-2.

I will get my work permit in May 2014. However, I want to file H1B in April 2014.

I thought that as long as you have an offer from a company , you can file H1B. Is this correct? 

I know that some people get job offers before they graduate, and filed their H1B before graduation since H1B has quota.