21606: The "From" phone number provided is not a valid, SMS-capable inbound phone number for your account.

You can only send SMS messages from a phone number or short code provided by or ported to Twilio, and that phone number must be SMS-capable. Visit the phone numbers page of your account portal to view a list of phone numbers that you own, and to determine whether they are SMS-capable.

Due to anti-spam regulations, the number a user sees when you send an SMS using your application must be the same Twilio number associated with that account. This differs from voice calls, where it is possible to display a verified caller ID other than your Twilio number.

Only certain countries and phone numbers are currently able to send global SMS messages. You may have received this error while trying to send from a number which is not capable of sending Global SMS messages. For more information, including an up to date list of countries that can send messages globally, please see our FAQ page on Global SMS.

If you received this error while trying to authenticate with your Test Credentials, you probably tried to send a message with a From number from your live account. The only number that can be used to send successful messages with your Test Credentials is +15005550006. For more information, read our documentation on the From number for sending SMS with your test credentials.