Hello Jaffer, did you have the device unlocked by Sprint and did you do a profile update prior to you leaving the US? If not, it sounds like this could be your issue. Typically, this is done prior to traveling outside of the US to allow the device to receive the unlock command.
Thank you for posting! To unlock your device for international use you'll need to contact Sprint Worldwide Care. Worldwide Care can be reached via chat or email from sprint.com/swwsupport or by calling 888-226-7212. Additional information on our international unlocking policy can be found at sprint.co/1jG1hW2