It will depend where you want to use the phone.
Within the EU, you can use your standard plan - there is a limit of 12Gb of data while roaming in EU.
Outside the EU, you will need to add 'credit' to your account to pay for any use. The rate will depend upon what country you are in. Best to look at the price guide for an idea as to how much 'credit' you may need.
https://cdn.smarty.co.uk/files/SMARTY-Price-Guide.pdf
