Hiring managers and HR people: when hundreds of people apply, how do you decide who to call or interview?

Basically looking at indeed, and they tell you how many people have already applied, and the numbers are very often over fifty! How many are a complete waste of time? I imagine you use software to look for keyword matches, how reliable is it? How does it grade resumes?

Keyword matching isn't all of it, but it's a part of it. With that in mind, I would spend time finding the important keywords in a specific job description and make sure you have as many of those words as possible in your resume.