Flipkart — Extramoney fraud by wishmaster

I purchased VIVO X70 pro plus and gave oneplus 6 as exchange on 31st October. Wishmaster took 4250 rs extra from me for the OnePlus 6. Amount for the exchange is 12700 and they took 4250 rs extra by saying this comes under damage category. But device which I gave was well maintained.
Later on I came to know these wishmaster doing same to many consumers. I called flipkart customer care and it's been 15 days now and they're not giving any postive reply. Now I am calling them and they're disconnecting calls again and again. This is clear cut fraud and I want a apology from Flipkart and my money back 4250 or my OnePlus 6 back.