Why is my SMS not arriving?

An SMS that fails to arrive almost always has one of five causes. In order of how often we see them.

1. The number is wrong

A typo, a landline in a mobile field, or a number without a country code. Sending internationally always requires the format with a country code, so +32 instead of a leading zero. Checking a list up front with an HLR lookup removes dead numbers before you pay for them.

2. The device is off or out of range

The operator then holds the message and retries. That can take hours. In the delivery report those messages show as Buffered: on their way, not yet delivered.

3. The sender name is refused

Some countries do not allow an alphanumeric sender, or require prior registration. The message is then blocked by the operator without the recipient noticing anything.

4. The recipient has opted out

Anyone who ever replied STOP is on the block list and receives nothing further. That is deliberate and you may not work around it.

5. The message expired

Every message has a validity period. If the device does not become reachable within it, the message disappears. For an appointment reminder that is fine; for a one-time code it is too long.

Always check the delivery report first: it tells you exactly which of these five cases you are in.