If it was bought from an online trader you have the automatic right to a full refund on any goods that you decide to reject (with a few exceptions, which aren't relevant here), irrespective of your reason (or complete lack of one) for returning the goods. So it makes no difference whatsoever as to how the phone was described or what condition it arrived in. [Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013].
If the phone was purchased from a private individual then it must be 'as described'. [Consumer Rights Act 2015]. Without seeing the item description that you responded to (which, from your post, did indicate that a fault existed), it's hard to know whether the phone you received was 'as described'.
If the person selling the phone was clearly (from their sales history on eBay or elsewhere) buying goods and then reselling them, rather than just selling their own things that they no longer need, the seller is legally a 'trader' (even if they're not registered on eBay, or similar, as a business seller) and my first paragraph above will apply. Otherwise you'll have to rely on the second one only.