I need help

Good day, I had an account with Blue label Telecoms and I took a year to get a cancellation letter. Then Blue label Telecoms sold the same account to Nimble and appears on my credit profile with Transunion, though I have sent a cancellation to them, so that they can update my profile. Now I cannot even dispute or ask for a phone call from transunion. My score went from 608 to 478 with days. Why? I have all the proof.