If you are going abroad you may look up the rules before you go.
Of course you will pay the difference if you are not on your home network. Your mobile number can not show any geographical information, you could be anywhere.
Do you think it would be fair for someone to call you not knowing you are away, and find when their bill comes in they paid a fortune; not because they did anything differently, but because you roamed away from home ? Obviously not, so it is only fair that if you roam away to a foreign country and choose to answer incoming calls you pick up the extra costs of an international call.
It would be in your contract.