The SIP header you asked Twilio to stamp was omitted because it is not supported on the Twilio platform.
Twilio supports extension headers beginning with an X- prefix. Your extension header name cannot begin with the prefix X-Twilio.
Possible Causes
SIP header name not supported on the Twilio platform.
Possible Solutions
If your application can use a different header name, use an X- prefixed header name, e.g. if you want to use the name Customer-Info, see if your application can be configured to use X-Customer-Info instead.