Summary
Occasionally a client may report they didn't receive an SMS survey invitation, even though our system shows successful delivery. This article explains why this happens and what can be done.
Background
SMS delivery has become increasingly challenging due to the proliferation of spam messages. Mobile carriers and handset manufacturers have responded by implementing more aggressive filtering to protect their customers. While this is generally a good thing, it can occasionally result in legitimate messages being filtered.
When we know a message wasn't delivered
When a message genuinely fails to deliver, our SMS provider reports a specific error. In these cases, we know the message didn't reach the client and can take appropriate action.
Common delivery failures include:
| Reason | Description |
|---|---|
| Unreachable handset | Phone may be switched off, in airplane mode, or out of signal range (usually temporary) |
| Message blocked | Recipient has opted out of SMS messages, or number is blacklisted |
| Unknown or invalid number | Number may be out of service, disconnected, or incorrectly entered |
| Landline | The number cannot receive SMS |
When these errors occur, we can see them in our logs and investigate accordingly.
When delivery is confirmed but the message doesn't appear
This article primarily addresses situations where our system shows "Delivered" status, meaning the carrier has confirmed receipt of the message, but the client reports they didn't receive it. The message was technically delivered but didn't reach the client's inbox.
Carrier-level filtering
From time to time, carriers may increase their filtering thresholds, which can affect delivery of legitimate messages.
When we get reports of SMS messages that are not received by clients yet show as delivered in our logs, we check the client's carrier and look for patterns with the same carrier. When we identify a pattern of SMS messages being blocked by a particular carrier, we work with our SMS provider to investigate and resolve the issue. This may involve contacting the carrier directly or adjusting our sending practices to improve deliverability.
Handset-level filtering
Even when a message passes carrier filtering, the client's phone may filter it into a spam or junk folder. This is especially common for messages containing web links from unknown senders.
iPhone users: iPhones have a setting called "Filter Unknown Senders" (Settings → Messages → Filter Unknown Senders). When enabled, messages from numbers not in the client's contacts are moved to a separate "Unknown Senders" list and notifications are silenced. The client may not realise the message has arrived.
Clients can check their phone's filtered or spam messages folder:
| Device | How to check filtered messages |
|---|---|
| iPhone | Open Messages → tap "Filters" in the top-left corner → select "Unknown Senders" |
| Android | Open the Messages app → tap the three-dot menu → select "Spam & blocked" |
If the client locates our messages in their filtered folder, they can mark the sender as "Not Junk" or add the sending number to their contacts to prevent future filtering.
Alternative channels
If SMS delivery continues to be problematic for your organisation or specific clients, we'd be happy to explore alternative channels such as WhatsApp. Please contact us to discuss options.
Reporting issues
If a client reports not receiving an SMS invitation, please submit a support ticket with the client's MRN and we can investigate further.
Identifying patterns across carriers helps us proactively address deliverability issues.
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