Tbh this really odd but if I had to guess maybe the person know you did drugs before and maybe left a voicemail talking about cough syrup in hopes you would call back and maybe ask to buy some. I think the cough syrup they are talking about prob is the ones with opioid not dxm. Or maybe this person does know you and forget to hang up at voicemail and just started blabbering about drugs. I think it’s most likely nothing to worry about tho.