Why is my Klaviyo welcome text not sending to new SMS subscribers?

There are three common causes to check in order. First, go to Klaviyo's settings, then SMS, then sending numbers, and confirm your sending number shows as 'Active.' If it says 'Pending' or 'In Review,' no messages can deliver at all and you need to contact Klaviyo support before doing anything else.

Second — and this is the issue that catches most people — check your welcome flow trigger. In Klaviyo, 'Subscribed to List' and 'Subscribed to SMS' are two different triggers that sound nearly identical. 'Subscribed to List' catches email sign-ups, not phone number opt-ins. If your welcome flow uses the wrong trigger, it will literally never fire for a single SMS subscriber. This has been called the sneakiest bug in the whole setup.

Third, test delivery across multiple phone carriers. Some delivery failures are carrier-specific, meaning a text that delivers perfectly on Verizon might vanish on T-Mobile. Test on at least five phones across AT&T, Verizon, T-Mobile, and one prepaid carrier. Also enable skipped profile notifications in your flow so you get alerted immediately when something fails rather than discovering days later that every new subscriber received nothing.