Unexpected behavior with SMS when user does not have a number

Our account has numbers assigned to Call Centers, but individual users do not have numbers assigned.

Within the Dialpad app, these users cannot send SMS to external numbers. The app has the message: "Messages to recipients outside of your company can only be sent from a call center." This makes sense, which is why the API's behavior was unexpected.

While working with the API, I was surprised that it allows requests from these users to the /sms endpoint, messaging external numbers. The API returns a 200, indicating that the SMS has been sent successfully, when in fact it hasn't.

Is there any way for me to know that these messages have failed?