You don't want to get lost in a big pile of applications so hopefully by being persistent, you will keep your name fresh in their minds. When you call, try asking if there are any updates on open PCA positions. Are there are any currently available, if they know when more will open up if they tell you there aren't any right now, etc. Anything to let them know you are really interested! Hospitals get tons of students applying for summer tech jobs. Maybe there would be more luck in trying to get a per diem tech job during the school year? Then you could pick up more hours during breaks. What about applying to hospitals outside the city, then applying to Boston hospitals after you have a little bit of experience?
Many of us have found that bypassing HR all together and going straight to contacting the nurse manager of the floor works better. Some hospitals might give you the name of the nurse manager if you call the main line. Do you have friends in hospitals that can get you some contact info? Your nursing instructors might have contacts at hospitals as well.
It's tough getting your foot in the door, especially in Boston, but something will come around eventually!