There are a few reasons why you cannot get signals from overseas:
- No Roaming subscription enabled. Check with your carrier to enable roaming. Note: Roaming for any cellular services can be quite expensive, much higher than your local service.
- Out of cellular coverage area for the preferred carrier for roaming. Network lock may be imposed by your carrier to ensure much lower roaming costs, and when you fall out of the coverage area of that carrier, you won't have service.
- No active roaming agreement in some countries. This happens more frequently for poorer Asian countries and many Middle East countries, as your carrier may have issues with the agreement due to coverage, costs or others.
- Different frequency bands and cellular technologies. Happens very frequently if you get a handset from US and go to Europe/Asia, especially when you get 3G handset from VZW/Sprint. LTE handsets for VZW may have GSM radios for international roaming, but still check with your carrier for supported GSM/WCDMA bands in the handset.