One of the first things to do when configuring A2Billing is set the base currency. This will be the currency that your rate cards are in. If you create call rates and then change the base currency afterwards, the rates will get messed up.
The first step is to edit the base currency setting. This can be found under SYSTEM SETTINGS / global / base_currency –
It is very important that you change the 3 letter value to a valid currency code. If you are not sure what these are then check in step 2 first.