Iphone 5c issue

So, here's the story:

Iphone 5c, bought it last week off Adverts, from a seller with 100% positive feedback. The phone was grand the first 24 hours and then it started..... Anytime I try to make a call or someone calls me I'd get a massive noise in the speaker, like static, the same for the caller.

Tried googling the issue, turning off data, resetting the phone, restoring it, no luck.

Has anyone here come across this problem and solved it permanently?
Is there any point going to Apple? I checked my IMEI and seemingly it's out of warranty.

Thanks.