This is probably the best piece of advice I can provide, to help you understand how to deal with employment agencies -> Employment Agencies in find people for jobs, not jobs for people
In other words – an employment agency is hired by companies, to find people for specific jobs. They are not in business to help you find a job and you are not their client.
So when getting in touch with agencies as part of your job search, make sure you state your interest for positions which are a very good match for your past experience/education. That will help you get more success from your dealings with a job agency.
It is also good to remember that relationships are important – People help people they like and know. Therefore, I would suggest calling an employment agency, after you have sent them your resume. The purpose of the call is to start developing a relationship with them and to get on their radar. Ask them when is a good time for you to explain your candidacy in more detail and let them know why you are well suited for particular roles you are applying to. In conversations with people, I often hear that employment agencies are rude, never call back, cancel/delay meetings and so on. Sure, some might be like that but not all. So I would still recommend getting in touch with them because you will have success with a few and they can be a good ally during your job search.