From the Travelsim ordering page:
The use of the Travel Sim card is expected to be a fair balance between free incoming calls and chargeable outgoing calls. Excessive use of free incoming calls will constitute abuse.
Found at:
Travel Sim Card - 100% Free Roaming .
BTW, I'm not criticizing the operator for that condition - I assume margins are thin, and I assume from this proviso that the margin is thinner (or perhaps even negative) on free incoming calls.