Unable to send text message to India (+91) using AWS SNS

This is what the CloudWatch response looks like

{
        "notification": {
            "messageId": "69f1ef66-3db1-5ed0-bbdf-b8b32c6dbe03",
            "timestamp": "2020-09-11 05:48:12.245"
        },
        "delivery": {
            "destination": "+91xxxxxxxxxx",
            "smsType": "Promotional",
            "providerResponse": "Internal Error",
            "dwellTimeMs": 41
        },
        "status": "FAILURE"
    }

I'm using the region ap-south-1 which is in the supported regions and using an Indian phone number (+91) but still getting this error. Apart from AWS CLI, I've also tried python boto3 library to send the message to no avail.This is the python code that I've tried:

import botot3
client = boto3.client('sns','ap-south-1')
client.publish(PhoneNumber='+91xxxxxxxxxx',Message='Hello')