Twilio error 21614 is an API-level validation error indicating that the destination To number is identified as a landline or non-mobile number that cannot receive SMS messages through standard SMS routing. This error is thrown before carrier submission when Twilio's number intelligence determines the destination is not a valid SMS-capable mobile number. The primary resolution is to filter non-mobile numbers out of your contact list before attempting to send.
What Causes This Error
The direct cause is submitting a To number that Twilio's number intelligence classifies as a landline, a fixed VoIP line, or another non-mobile number type that lacks SMS reception capability. Business phone numbers, home landlines, and corporate PBX extensions make up the most common sources of 21614 errors in B2B and mixed-use contact lists. Numbers collected from older CRM systems or paper-based records, where no distinction was made between mobile and landline numbers at the time of collection, frequently generate 21614 because a significant proportion of those numbers are landlines. Toll-free numbers in the destination list that have not been provisioned for SMS reception also return 21614, as receiving SMS on a toll-free number requires specific carrier provisioning that not all toll-free numbers have.
How to Fix It Step by Step
Use the Twilio Lookup API with type=line_type_intelligence on every destination number that returned 21614: call GET v1/PhoneNumbers/{number}?Type=line_type_intelligence and examine the line_type field in the response, which will show landline, voip, mobile, or other classifications. Move all numbers classified as landline, non-fixed-voip, or voip to a suppression list in your database and remove them from your SMS send queue. For contact records where you need to maintain communication despite the number being a landline, implement an alternative outreach channel such as a voice call (which landlines can receive) or an email if the contact also has an email address on file. Update your database schema to store the line_type classification alongside each phone number so that future send jobs can filter non-mobile numbers without re-querying the Lookup API.
How to Prevent It from Recurring
Validate the line type of every phone number at the point of collection using the Twilio Lookup API with line_type_intelligence, and store mobile-only contacts in your SMS contact list while routing landline contacts to alternative communication channels. For large existing contact databases that have never been validated, run a batch Lookup job to classify all stored numbers and populate the line_type field, allowing your send logic to skip non-mobile numbers without making a live Lookup call per message. Add a pre-send filter to your messaging pipeline that checks the stored line_type for each recipient and skips numbers with non-mobile classifications, preventing 21614 before the API call is ever made. Schedule monthly re-validation of your contact database using the Lookup API, as numbers can change type when subscribers port between providers, ensuring your line_type data does not become stale.
When to Call a Specialist
If the Lookup API classifies a number as mobile but Twilio returns 21614 when you attempt to send to it, there may be a data inconsistency between the number intelligence database and the routing database that requires Twilio support to investigate. A specialist can also help you design a contact lifecycle management system that handles the full range of number types, maintaining correct routing logic as contacts change from mobile to VoIP to landline over time. For B2B applications where your contacts frequently use VoIP numbers that technically could receive SMS but Twilio classifies as non-mobile, a specialist can assess whether alternative routing through MMS or other channels could improve reachability for that contact segment. High 21614 rates in your send logs are a data quality signal that should trigger a full contact list audit, which is most efficiently conducted with specialist tooling.
Conclusion
Error 21614 is a number type mismatch that is resolved by filtering non-mobile numbers from your send list using Lookup API line type intelligence before the API call is made. If this error is blocking your production system, contact our team and we will diagnose and fix it within the hour.
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