How do I start a return?

I tried to start a return this morning via the webchat but "Wendy" didn't really help me and was quick to close the chat.

2 days ago I purchased a pay monthly plan, however I was informed that the payment didn't go through properly and didn't receive proper confirmation. The following day I raised another order and later that day I found out that the original order had in fact gone through. I tried to cancel one of them before it got dispatched but nobody was able to do that via the webchat so I now have 2 phones and need to send one back! 

Someone help me please! I need to know where I'm sending it, which direct debit am i  canceling, whether I'm going to have to pay for postage etc. 

My order number is 114426402 & the phone number which I'm sending back is 07449590143. I also have the chat saved with Wendy if anyone would like to investigate the "assistance" I received.