For the most part, your international callers will be charged local call rates if they are using a local dial-in number for their country. We always suggest that participants check with their telephone provider as to the charges (if any) that they may incur. In any case, charges for local calls will invariably be much cheaper than having them make an international call.
For example, for calls within the UK from a fixed line or mobile to the UK 03 Dial-In Number, callers will be charged the same rate as they would for other standard 01 or 02 fixed line numbers. This means that they will be counted as part of any inclusive or free minutes allowances.
Please see this support article for more information: Call rates and charges explained