Issue: Unable to Configure SNS Topic for End User Messaging (2-Way SMS) – IAM Client Config Error

This error indicates an issue with the IAM client configuration when trying to set up an SNS topic as an event destination for your two-way SMS messaging in AWS End User Messaging.

To resolve this issue, you need to ensure that the proper permissions are in place for AWS End User Messaging SMS to publish to your Amazon SNS topic. Here are the steps to fix this:

1. First, verify that your Amazon SNS topic has the appropriate resource policy (access policy) attached to it. The policy should include a statement that allows the "sms-voice.amazonaws.com" service principal to publish to your SNS topic.

2. Update the access policy on your SNS topic with a statement like this:

{
"Effect": "Allow",
"Principal": {
"Service": "sms-voice.amazonaws.com"
},
"Action": "sns:Publish",
"Resource": "arn:aws:sns:[REGION]:[ACCOUNT_ID]:[TOPIC_NAME]",
"Condition": {
"StringEquals": {
"aws:SourceAccount": "[ACCOUNT_ID]"
},
"ArnLike": {
"aws:SourceArn": "arn:aws:sms-voice:[REGION]:[ACCOUNT_ID]:configuration-set/[CONFIG_SET_NAME]"
}
}
}

3. Replace the placeholders with your specific values:

• [REGION] with your AWS Region
• [ACCOUNT_ID] with your AWS account ID
• [TOPIC_NAME] with your SNS topic name
• [CONFIG_SET_NAME] with your configuration set name

4. If your SNS topic uses server-side encryption with AWS KMS, you'll need additional permissions on the KMS key policy to allow the sms-voice service to generate and decrypt data keys.

5. Try creating the event destination again after updating these permissions.

If you're still encountering issues after updating the policies, you may need to contact AWS Support for further assistance with this specific error message.
Sources
Amazon SNS access policy - AWS End User Messaging SMS
IAM policies for Amazon SNS topics - AWS End User Messaging SMS
Topic policies for Amazon SNS topics - AWS End User Messaging SMS