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That did the trick! Thanks Ilya!

Brandon,
Please check that your Content-Type header equals to "application/json". If equals, please specify full request you are sending to server.

Thanks for that! I tried updating the message, but got this error:

{ "message" : "Internal Server Error. Consult RC Support.", "eventId" : "eaae36c7-8d7e-410a-97d2-1e8349266a6f" }

Any idea what might be wrong?

To update a voicemail or other message's "readStatus" property, use the Update Message API for the messageIds desired.

This API is documented in the Developer Guide API Reference here:

https://developers.ringcentral.com/api-docs/latest/index.html#!#RefUpdateMessage.html

You can also try this out on the Sandbox Environment using the API Explorer below (don't forget to authorize):

http://ringcentral.github.io/api-explorer/#!/Messaging/v1_0_account__accountId__extension__extension...