Cognito User Pool custom SMS sender

Hello,

I'm trying to customise a particular User Pool (let's call it UserPoolA) to send text messages (eg, sms) from a particular number, instead of the default one.

Requirements:

1. all other sms activity from our account needs to stay as is (eg, NO changing the default sms settings).

What works:
The user pool is configured properly, I can signup with a phone number and I get a text message with the verifying code. Then I can login with my phone number (and password) and go on with my day.

What I tried already:
Following https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-sender-triggers.html I tried to set up a custom sender trigger, so I can then send messages from code via sns. Obviously, instead of actually sending messages I logged some stuff to stdout (console.log() for js and print() for python). This doesn't work. I do get my debug messages in CloudWatch if I call the lambda function via the Test functionality in the lambda console, but when I try to signup (via the hosted UI, remember this works with the default Cognito sender) I only get an error message "An error was encountered with the requested page." and the url is: "https://<foo>.auth.us-east-1.amazoncognito.com/error". (Obviously, I'm deleting my existing user before trying to signup again).

Question:

1. is there a way to get logs from Cognito about when/if it tries to call my CustomSender trigger?

2. is there a more detailed explanation about the CustomSenders than the link I posted?

Output from describe-user-pool after I add my CustomSender lambda:

"LambdaConfig": {  
    "CustomSMSSender": {  
        "LambdaVersion": "V1_0",  
        "LambdaArn": "arn:aws:lambda:us-east-1:...:function:CustomSMSSender"  
    },  
    "KMSKeyID": "arn:aws:kms:us-east-1:...:key/xxx-xxx-xxx-xxx-xxx549"  
},  

Lambda policy:

{
"Version": "2012-10-17",
"Id": "default",
"Statement": [
{
"Sid": "CognitoLambdaInvokeAccess",
"Effect": "Allow",
"Principal": {
"Service": "cognito-idp.amazonaws.com"
},
"Action": "lambda:InvokeFunction",
"Resource": "arn:aws:lambda:us-east-1:...:function:CustomSMSSender"
}
]
}

Edited by: pbalau on May 6, 2021 8:53 AM

Edited by: pbalau on May 6, 2021 8:54 AM