My example was about retail prices: You're correct that due to clearinghouse there are hardly any charges exchanged - as in most cases the outgoing roaming volu

My example was about retail prices: You're correct that due to clearinghouse there are hardly any charges exchanged - as in most cases the outgoing roaming volume of a provider is comparable to the incoming roaming volume. But if you have two countries where retail prices differ a lot, this can lead to people from both countries to buy SIM cards from the cheaper country, causing an asymmetry in the roaming volume: Now suddenly one provider has lots of outgoing roaming, but much less incoming roaming - which means that this provider is going to have to pay the wholesale prices for the difference in roaming volume.