How to go about with a credit dispute???

Hello everyone! I'll try to keep my situation short and simple. I recently ended a contract with my cell phone provider early. I had two lines in my name. After ending it, I received a notice in the mail from a collections agency that I owed the cell phone company $1,200.00 they informed me that I had 30 days to dispute the charges. I sent the CA a letter informing them that i disputed the charges and wanted them to validate my debt. It is about one month and two weeks later, and I have heard nothing from them. At the time I sent the letter, I was unaware of the concept of debt validation. After reading on it, I see that I should have sent the letter through certified mail and got a receipt. Can anyone please inform me on what I should do now? Would it be beneficial to still dispute this to the CRA without a receipt? Also, I do have the original copy of the letter. Thanks in advance!!!