Android phone locked to a carrier? Is it easy to unlock?

I was planning to get a Samsung S4 using SMART's unlimited data plan as a gift for wifey.
Wifey is a Globe subcriber so the plan is just to give her the phone and I will use the SMART sim since I am a heavy user of SMART 3g.

But SMART says the phone is locked to them? First time to hear an android phone locked to a carrier.

Is it easy to unlock android phones? I've search the internet but I want to hear some personal experiences.

Thanks!