It is a unique identifier used by some mobile network operators to facilitate mobile number portability. This allows users to retain their mobile telephone number when switching operators.
Data roaming is when you are using data on a mobile network abroad. This could incur unexpected charges.
The telecoms companies available to Islanders are Jersey Telecoms (JT), Airtel-Vodafone and Sure.
One of the main ways to reduce billing could be by switching to a more suitable mobile package/plan. You may be paying for extra data or minutes that you do not need.