Your bill will include your plan fee for the month in advance along with any charges for out of plan usage from the previous month.
Out of plan Usage includes:

• International Calls/Texts
• Calls to premium numbers (including 11811)
• Rest of World Roaming (Roaming outside the EU)

All of your bills are itemised, you c an see a full breakdown of all charges by looking at the PDF version of your bill. Your bills are available to view in your MyGoMo account.