The header you asked Twilio to stamp was omitted because it contains illegal characters in its name. Including this header would have resulted in Twilio generating a message that runs contrary to the SIP specification.
The set of legal characters for a header name is defined in RFC 3261.
- If you specified headers in a SIP URI, invalid characters were most likely introduced by an escaped character. Escaped characters in SIP URIs are defined by a percent sign (%) followed by two hexadecimal digits specifying a character code.
- Ensure that your SIP URI is formed correctly. Header names should generally only consist of alphanumeric characters and hyphens.