SMS: unsendable numbers

When a provider tells us that a phone number can't receive SMS, we stop sending SMS to that number so you don't pay to text phone numbers that can't receive your messages.

Some phone numbers can’t receive SMS: they’re invalid, they belong to a landline, or the handset can’t be reached. Every attempt to send to these numbers costs you money, drags down your sender reputation, and clutters your delivery metrics with bounces.

To prevent this, Customer.io watches for SMS bounces. When a provider tells us that a phone number can’t receive SMS, we flag the profile and stop sending SMS/MMS to that number. The way we handle bounced SMS messages is very similar to the way we handle SMS opt outs.

How it works

When an SMS bounces with a provider error telling us that the number can’t receive SMS, we set a reserved cio_sms_unsendable_reason attribute on the profile. The value records why the number can’t receive SMS:

The attribute also tells Customer.io to drop future SMS to that person’s phone number. We record these messages with the dropped metric, the same metric we use when we skip messages for opted-out recipients. Dropped messages don’t count as bounces and don’t cost you a provider request.

Dropped SMS messages don’t affect other message types

The cio_sms_unsendable_reason attribute is tied to the profile’s phone attribute and only applies to SMS messages. Customer.io will still send other kinds of messages to the user—email, push, in-app, etc.

WhatsApp bounces don’t set or rely on this attribute. WhatsApp bounces don’t include information about why they bounced, so we don’t apply or observe the cio_sms_unsendable_reason attribute for WhatsApp messages.

If you use a different attribute to represent the phone number, the cio_sms_unsendable_reason attribute doesn’t apply. For example, if you store a preferred_phone attribute and try to send to that rather than phone, any change to the cio_sms_unsendable_reason attribute won’t apply.

Customer.io automatically clears cio_sms_unsendable_reason when a phone number changes

If you update a profile’s phone attribute, we’ll clear the cio_sms_unsendable_reason attribute so the new number can receive SMS messages. You don’t need to remove the cio_sms_unsendable_reason attribute yourself.

Find and segment unsendable numbers

You can segment on the cio_sms_unsendable_reason attribute like any other attribute. This can help you find profiles that can’t receive SMS. You might do this either to exclude them from your automation workflows or to clean out inactive profiles. For example you might create a segments with the following conditions:

• Segment where cio_sms_unsendable_reason exists to find every profile with a number that can’t receive SMS.
• Segment where cio_sms_unsendable_reason equals landline to find profiles whose number is a landline, so you can reach them on another channel instead.

cio_sms_unsendable_reason is a reserved attribute that Customer.io sets and clears for you. You can’t set or edit its value yourself.