Instalment deduct after piad in full

I requested a settle amount last month and was given a amount that must be paid before 24/*** which I paid on the 23rd and mail them the proof of payment as instructed. They then deducted a installment on the 27.03.2021. I tried to contact them via phone, emails and their website with no luck. The agent either puts the phone down in my ear or they don't have access to the account or they transfer me to multiple agents who does not work with accounts. Their website is under construction...etc... All I want is proof that my account is paid up and my instalment back.