Replacement iPhone operator lock.

Hello,

About 11 months ago I bought a sim-locked iPhone from my local operator.
Last march I got 2 replacement iphones (first came with dead pixels, Apple sent me a second one that worked fine).

However, this third iPhone is really strange regarding the sim LOCK.
So, it's supposed to be locked to my operator (Movistar Spain).

If I put any other spanish sim card (Vodafone, Orange), the iPhone asks to be activated thru iphone, and the process logically fails.

HOWEVER; If I put my old italian sim card (from TIM, one of the official carriers in Italy that sell the iPhone), I can successfully activate it thru iTunes.

That means that I can use my TIM simcard and my MOVISTAR simcard in a phone that is supposed to be sim-locked to movistar.

Moreover, I borrowed a friend's iphone, also locked to movistar, and I put in my italian sim card. The iphone directly said "incorrect SIM" it didnt' ask for activation. So I reckon there's something different about this replacement iphone since it doesn't reject other sim cards (never shown "incorrect sim") but activation fails.

Any explanation for this?
(I've been told maybe I was given a replacemtn iphone meant for the italian market where iphones are sold unlocked - but as I've said, my iphone is not unlocked to all operators, only to TIM. I guess I would need to try a Sim card from another country that sells iphones but I don't have any at hand).