There are several Content-Types Twilio accepts. If Twilio cannot process the URL's Content-Types, it results in error 12300. Stating a Content-Type for a request is essential; otherwise, it would result in a 502 (Bad Gateway) error.
For example, attempting to play non-audio content like XML by having a Play verb is a cause for Twilio error 12300.
Read more about accepted Content-Types here.
It would help if you verified that your web server returns the expected value as the Content-Type, and the URL refers to a valid and acceptable resource. In addition, you have to determine that the messages going to and from the Twilio phone number utilize a supported Content-Type.
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