Petr is on a trip in Germany, and he calls his friend Thomas, who has a German mobile. Petr pays the rate charged on his roaming tariff (e.g. with O2 Eurotarif he will pay CZK 12.60 for a 2-minute call in Germany).
If Thomas calls Petr, Petr will pay for the incoming call according to his roaming tariff (e.g. CZK 3.30 for 2 minutes call with O2 Eurotarif).