What Actions Should I Take To A Collections Agency For Being Charged For A Debt That I Already Paid For?

I have paid my $1500 debt to a Collections Agency 1 that T-Mobile referred my delinquent amount to. Yet, after i have paid debt in full, another Collections Agency claims that they have no record of my payment. I have send them five copies of my moneygram receipt, non certified mail, and still get calls from the Collections Agency 2. I get the usual run around from the collector that they have not receive my proof of payment. There is no way that a mailman loses 5 letters. What actions should i take to make this correction? I am tired of their calls. It is stressing me out that the collections agency isn’t budging. Now is becoming harassment
What I’ve done so far:
State to the collector that I have made a full payment to the previous Collection Agency 1. Collector requested a hard copy of my payment and I did, 5 non certified mail to the Collections Agency 2 address. Collection Agency 2 claims that they have not receive the letters, and that is best to send the original hard copy, which i find absurd because if the original copy gets lost somewhere in space, then there goes my proof.
What actions should i take? Yes it is simple changing my cell phone number, but this is affecting my credit score. I’m am trying to get my credit score back up, I have learned from my mistakes, paid my debt, yet I am being charged again from another company.
I will post the two Collections Agency Company names later when I review my documents.