What to do when financial collection agency starts to calling me?

The debt is for a phone company from my past cell phone. The reason I didn't pay is I received some useless text messages which I didn't pay attention for, and later they charge me for $60 for that. I feel deceived so I refused to pay. Now NCO financial starts to calling me and I don't know what to do. I wonder what would happen if I do not pay for it? Shall I tell them stop calling me?