Some users complained they are unable to reply to the SMS. For Singapore numbers recipient, they are expected to reply to a 5-digit short code number. A 5-digit short code number is also regarded as "premium" SMS in some cases even though we do not charge the recipients for the incoming SMS.
Possibility 1: Recipient has turned off Premium SMS settings
Firstly, find out if they are using some smartphone models that may have settings to prevent sending of SMS to premium SMS. Here is a blog which shows how it can be turned on.
e.g. Samsung models
Possibility 2: Carrier Delivery Issue
If the first issue is eliminated, report the mobile number of the affected recipient to our support and we will check with the carrier. Please allow us at least 3 working days to perform the investigation with the carrier.