If you are an Apple (iOS) user, these charges come from the activation of iMessage and FaceTime. Both services involve incurring a cost for sending an international SMS message. This SMS is sent in the background by your iPhone (to Apple's servers in the UK) when setting up these services. For all international SMSes originating from Singapore, redONE charges a $0.15/SMS sent. Roaming charges will apply if the SMS is not sent out from Singapore.
Moreover, when a user tries to activate iMessage, a popup message will appear citing that the carrier may charge for messages used to activate iMessage.
From time to time, Apple may prompt out a message to reactivate the iMessage or Facetime service. Clicking OK will allow Apple to send another international SMS message to the UK servers again. Some situations where Apple may prompt out the reactivation message are when you swap SIM Cards, lose network connectivity, or enter flight mode for an extended period of time and reconnect.
This is an Apple service, and to prevent these charges from happening in future, we advise that subscribers no activate the iMessage and Facetime functions.