Revocation of o2 My Handy Contract

I had ordered a phone (Ratenplan) through the o2 Website on 19.08.2017.The phone arrived on 22.08.2017 Since the phone turned out to be a single sim variant as opposed to the dual sim variant (no concrete information on the website) , I decided to return the device back (containing the Retoureschein) within 14 days, i.e. 31.08.2017. I also faxed as well as emailed o2 the revocation form but still haven't received any confirmation from their side. Any idea how long does it take to receive the confirmation ?