Making voice contact with the hiring manager before applying for a job is a recommendation I make to all job seekers because it is the most effective way to get an interview and job.
But think before you call to request an informational interview. Imagine you are the hiring manager and you get an unsolicited call from someone who tells you he is looking for a job and would like to set up an informational interview. What would your immediate reaction be? Most managers will keep the caller at arm's length and say something to disengage from the call. Introducing yourself by asking for an "informational interview" or saying "I'm job searching" is a big turn-off. It puts the recipient into a negative posture. Only directly asking for a job is worse.
If you have been referred to the hiring manager, making a call is quite easy: "Hello Ms. X, my name is Y and I was referred to you by Z." Then ask your questions about the position you have been referred to and the problems to be resolved. Knowing these enables you to relate some of your accomplishments that would help.
But if you have not been referred, a far better approach is to establish rapport and finesse the discussion without mentioning turn-off words like interview, job searching or job. I suggest starting by asking a question about something of interest to the hiring manager. That takes some research and preparation. You could Google the manager as well as the company for information and check the manager's profile on LinkedIn. You will almost always find something the manager is interested in or something new about the company to ask questions about. Do this before there is any discussion about an open position or your search.
With rapport established, keep the focus on the manager by mentioning something about the company and his position in it. You want to learn about problems he or she faces so you can mention that you have resolved those kinds of problems, again without indicating you are job searching. You want to get the hiring manager to start asking questions about you. Ultimately you'd like him or her to suggest an interview because that gives you the opportunity to casually indicate you'd be happy to do so. If the manager does not suggest an interview, then it is time for you to indicate you would be interested in talking to him or her about opportunities (even though you may already know there are some). So the only time you should suggest an interview is if the manager doesn't initiate it.
The takeaway is this: Making voice contact with hiring managers is a very effective job search tactic. Being referred to the hiring manager is hands-down the best situation. Avoid mentioning anything about job searching. Finesse the conversation; get the hiring manager interested in you without mentioning your situation. Look for opportunities to demonstrate your ability to help resolve the hiring manager's problems. Strive to get the manager to suggest an interview. And remember that people are busy. Your call is probably interrupting the manager, so be well prepared, brief and to the point.