As of December 1st Singtel will be blocking any alphanumeric/short code source addresses that have not previously registered with them in preparation for the new DNC Singapore regulations that will go into effect in on January 2nd 2014.
To avoid service interruptions we recommend that you change your messages to only use numeric originators prior to this date. To assist you with this, we will automatically overwrite alphanumeric / short code source addresses with a default numeric in order to aid delivery, but it is recommended that you should use a numeric source address of your own choice which is preferably a World Text virtual number.
For more information about the new regulations please see the information at http://www.pdpc.gov.sg/organisations/do-not-call-registry-your-business