Invalid "To" Phone Number error, or the error 21211, is an error occurring when the number you supplied when attempting an outbound call is incorrectly formatted or invalid. Another cause for this error is that you tried to call or send a message from a certain Twilio number to itself.
To solve this issue, ensure the phone number is in the E.164 format and the correct country calling code is applied to the number you are trying to reach as given below.
[+] [country code] [subscriber number including area code]
Make sure the caller and callee are two different numbers, and you are not attempting to place a call to the same Twilio number.
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