Why are my messages rejected with reason code 53?

Messages will be rejected with reason code 53 when for the recipient of the message an invalid mobile number is set. Our API requires recipient numbers to be in international format without leading + and 00. What does this mean? This means that a number starts with the country code followed by the national number, without leading zeros.

Example: if your national number would be 08123456789 and the country code is 65, the number would be 658123456789.

The most common reasons of an invalid mobile number are:

  • The recipient number is not prefixed with the country prefix. Example: a mobile number in Singapore would be 8123456789 but this is without the country prefix. So to make it a valid mobile number in international format, we should add the country 65 to it, so 658123456789.
  • The recipient number is prefixed with the country prefix but the leading 0 of the national number is not removed. Example: if the national number is 08123456789 and you add the country prefix 65 to it, you should remove the leading zero of the national number and then prefix with the country code, so 658123456789.
  • The recipient number is a fixed line phone number. Although these are valid phone numbers, SMS messages cannot be send to such numbers.
  • The user entered some random digits. When a user enters a mobile number, you can use our Number Lookup service to check if it is a valid mobile number.

If you believe the mobile number is correct and your message is still rejected with reason code 53, please contact our support.