Galaxy S5 :: How To Find Out What Network Phone Is Locked To

I bought a Galaxy S5 from Argos very recently since I needed a sim free phone, it was a gift for my brother who was moving abroad. However, he has since informed me that the phone is locked and is not accepting the local sims. Now I feel this may have been my fault because I've heard you need to use a couple UK sims before going abroad which I never did. Anyway, I wanted to grab an unlock code for him to use but all the websites ask what network the phone is locked to, which he can't find since it was originally unlocked from Argos.

So in summary, I need to somehow find out if the phone is locked to a network even though the phone was bought unlocked. Pretty confusing, I know.