Unlocked iPhone not working with other sims?

So, my iPhone 5 which was bought from carphone warehouse and they state that all their phones are unlocked and they confirmed mine was unlocked. On my holiday to india, I tried many different networks and it says 'sim not valid' what's wrong here? Is carphone warehouse just lying and is there a way to get around to it next time I'm abroad?

You should tell them this situation.

Usually when you buy an unlocked phone, the operating system is meant to not take SIMs. It's "factory unlocked" so you can't. Sorry

Probably wasn't unlocked correctly. Once a phone is correctly unlocked then it accepts all sims