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You need to provide a snippet of you text what you are trying to send. There should not be any issues in a normal case if you text is within the allowed range.

Without getting a snippet of your text it's very difficult to assume what is going wrong.

Here is a very basic sample to API body of SMS API :

Body:

{"from":{"phoneNumber":"<your phone From number>"},"to":[{"phoneNumber":"<your To phone number>"},{"phoneNumber":"<your To phone number>"}],"text":"Test sms test"}

Please provide your complete request and response (along with headers).

Have you tried using just a simple text string? The "text" property must be: Text of a message. Max length is 1000 symbols (2-byte UTF-16 encoded). If a character is encoded in 4 bytes in UTF-16 it is treated as 2 characters, thus restricting the maximum message length to 500 symbols.

You can see the documentation for this API resource here: https://developers.ringcentral.com/api-docs/latest/index.html#!#RefSMSMessages.html