ATT Prepaid - why I paid Deposit 3 times the price for a Certified Refurbished Phone ??

I got a phone from ATT, there seemed to be some issues with with voice.  so I requested for a replacement, a $122.30 some dollars I had to pay for some reason to send in a replacement phone.  I was surprised ATT asked me for deposit because the phone was free as part of promotion going on on prepaid network for certified refurbished phones and barely worth $40 otherwise.  I was total clueless, but I paid none the less the hefty amount.  I shipped the same phone I received, back within 10 days of receiving the phone, when will I get the refund back.  As per USPS, the phone was received by an Agent on Aug 14, 2018.  

by the way I returned to you the same phone you sent me as the phone I had was working fine later on... probably some of your network towers repairs going on or something so I had problem earlier..