REST API Unicode characters not working

I've been able to reproduce this in a test environment using Workbench. It appears that Salesforce by default interprets text in the body of a request with Content-Type: application/x-www-form-urlencoded as either ISO Latin-1 or UTF-16, which is why that specific piece of text yields an eth character (Unicode/Latin-1 F0).

If you can get Twilio to send the header Content-Type: application/x-www-form-urlencoded; charset=utf-8, I've confirmed that this fixes the issue.

Unfortunately, it looks like Apex grabs the request body and deserializes it to params before we actually ask for params, and the original binary request content is no longer available by the time it gets to us. Further, setting the Content-Type header on the RestRequest object once it gets to Salesforce has no effect.