Lack of communication

I had an outstanding balance of R1143.69 with Fasta loan, I emailed them requesting a payment plan for 3 months, then i started making payments, after making my second payment i was then contacted by CORE CONNECT saying that my account was handed over to them with the balance of R761.69, and if we minus 382*2 from the R1143.69 my balance should be R379,69 but the CORE CONNECT people are saying i should pay R761.69 and this account was handed over to them 3 days after my last payment to fasta and they cant allocate my last payment. Fasta should update my account with them but i have been contacting them by *** no one is responding to me and i keep getting calls form CORE CONNECT they say they cant help me fasta should be the one to help me.