XOIP credits are what you use to pay for sending faxes or forwarding incoming phone calls. You can purchase credits in your XOIP account through making an online payment. The credits are 1:1 denominated in Euros.
The same XOIP credits can be used towards sending faxes and towards forwarding incoming phone calls.
When you send a fax or a call is being forwarded, the associated costs are automatically deducted from you XOIP credit balance.
When logged in to your XOIP account, a credit indicator will show you your credit balance at any time.