I had the same issue when I traveled to Singapore. It seems they enable it automatically for 8 Asean regions regardless. As long you exceed a threshold of RM10.00 per day via Call( called/received) or SMS. Sometimes it doesn't make sense cause I receive call from Maxis only not even more than 1 minutes or less but still get it activated automatically and they mention it exceed RM10.00 so automatically actives the service. The most annoying is that the option they gave me after coming back from overseas call to cancel roaming and subscribe again when travelling..

FYI as long your Line appears as overseas Telco with Data Mode off.. Their trigger will be call and sms.