If you are applying online to a big tech company, it almost always goes into an Applicant Tracking System (ATS). The ATS scans your resume and cover letter for keywords, and matches it with the keywords in the job description or specific keywords as asked my the hiring manager. (You can get through ATS just by copy-pasting the job description in your cover letter. Don't do it). Once you pass through the ATS filter, the recruiter don't seem to care much about the content of your cover letter, but it makes a huge difference when it goes to the hiring manager.
Apart from convincing why you are a perfect fit for the role, share interesting stuff about you like a link to your website (highly recommend this for new grads in tech roles), github profile, previous internship experiences and what excites you about this role.
PS - The most effective way to get a call is to network.(and avoid the whole ATS blackhole).