Twilio error 63003 means WhatsApp could not locate an active account associated with the destination phone number you specified in the To field of your API request. The WhatsApp Business API channel performs a lookup against WhatsApp's user registry before attempting delivery, and when the lookup returns no matching account, it returns this error immediately without queuing the message. This is distinct from a delivery failure: the message never enters the WhatsApp network because the recipient address does not resolve to a registered WhatsApp user.
What Causes This Error
The most common cause is sending to a phone number that the recipient has never registered with WhatsApp, meaning the number exists as a valid phone number but its owner has not installed or activated the WhatsApp application with that number. A second cause is providing the To number in an incorrect format: the WhatsApp channel via Twilio requires the number in E.164 format prefixed with whatsapp:, for example whatsapp:+12125551234, and passing a number without the whatsapp: prefix or without the country code causes the lookup to fail with 63003. Numbers that were previously registered on WhatsApp but have since been deregistered, either because the user switched devices, ported the number to a new carrier, or explicitly removed the number from WhatsApp, will no longer resolve in WhatsApp's registry and return 63003. Landline numbers that the recipient may have attempted to register on WhatsApp using the voice verification method but never completed the activation process appear as incomplete registrations that WhatsApp's routing layer does not consider valid delivery targets.
How to Fix It Step by Step
Open the Twilio Console and navigate to Monitor, then Logs, then Messages to locate the failed message record, which shows the exact To value your application sent along with the 63003 error code returned by the WhatsApp channel. Verify the To field value in your application's outgoing message request: confirm the format is exactly whatsapp:+[country code][subscriber number] with no spaces, dashes, or parentheses in the number portion, and that the country code is present and correct. Ask the intended recipient to confirm they have WhatsApp installed and active on the destination number, and have them send your business WhatsApp number an initial message, which both opens a session and confirms the number is registered. If you are sending to numbers sourced from a CRM or database, run a batch validation using the Twilio Lookup API with the type=whatsapp parameter to identify which numbers in your list have active WhatsApp accounts before attempting delivery, filtering out numbers that the Lookup response classifies as not having WhatsApp.
How to Prevent It from Recurring
Integrate a WhatsApp reachability check into your lead capture or contact import workflow: when a new number enters your system, call the Twilio Lookup API with the WhatsApp channel type and store the result in the contact record, flagging contacts as WhatsApp-reachable or not at the point of ingestion rather than at the point of sending. Build your outbound messaging logic to check the stored WhatsApp reachability flag before constructing a WhatsApp message request, routing contacts without confirmed WhatsApp accounts to an SMS fallback path instead of attempting a WhatsApp delivery that will fail with 63003. Add the To format validation as a pre-send assertion in your code: assert that every value passed to the WhatsApp channel starts with whatsapp:+ and contains between 8 and 15 digits after the prefix, which catches formatting errors before the API call is made. Monitor the 63003 error rate in your application's metrics dashboard and alert when the rate exceeds 5 percent of outbound attempts, as a sudden spike may indicate a data quality issue in a recently imported contact list.
When to Call a Specialist
If you are consistently receiving 63003 for a number you are certain has WhatsApp installed and active (confirmed by the recipient), there may be a regional lookup delay where WhatsApp's registry has not yet propagated a recent registration, and a specialist can verify whether a retry after 24 hours resolves the lookup or whether a deeper platform issue is at play. A specialist is also valuable when you are building a large-scale WhatsApp outreach system where validating millions of numbers via the Lookup API is cost-prohibitive, and you need an alternative validation strategy that balances accuracy against API costs. You should also escalate if 63003 errors are appearing on numbers that previously received WhatsApp messages successfully, as this can indicate the recipient has deregistered their number and your contact database needs to be updated, which a specialist can automate using StatusCallback data.
Conclusion
Error 63003 is a recipient lookup failure that is resolved by validating the To number format, confirming the recipient has an active WhatsApp account, and implementing pre-send reachability checks via the Twilio Lookup API. 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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