CPC Payment Made OOPS

I was contacted by Central Portoflio Control, my original credtor was Heritage Chase. I have already made one of the three payments they have requested. Is that admission that the debt is indeed mine? I called the 1800 number they listed and was harassed, belittled and brought to tears even before I got my question to the man out. I am considering sending a Debt Validation letter to them. Is it useless now that I have already made one payment? Can I just not send in the other 2 payments they requested? The worst part of it all is that now they have my cell phone number! I am trying to take care of old credit cards from when I was in college (6 years ago), and this is one of them. I mean if I already have a low credit score what is the point of paying them?