Manufacturing defect iPhone 4s

Last 23 July 2012, I bought an iPhone 4s from Movistar Spain, the iPhone came with a manufacturing defect, everybody who calls me listen to them with echo, at first, for a couple of days, I throught it wasn't my problem but when everybody complained about the echo, I phoned to Movistar Spain, 1004, in order to get a new one. That day started my nightmare.

Movistar says they usually change the devices but this case is different because it's an iPhone so I must phone Apple, Apple says that Movistar must change the device, and so on. Movistar and Apple want to repair it but I paid for a NEW iPhone, I haven't bought a repaired one.

Meanwhile the time I have to get the iPhone changed is going to expire.

Please help me! What can I do? I went to two different Movistar shops they told me that they can't do anything because I bought it throught a telephone offer (1004), there isn't any Apple Store near my city.

When I threatened Movistar to go to Orange or Vodafone they promised they changed the iPhone but two days later they phoned me to say that's was impossible.

I think Apple must solve this problem because it's a manufacturing defect and Movistar is one of the companies Apple uses in Spain to sell the iPhone.