title: Problem with Boto3 SNS SMS publish with OriginationNumber
body: <p>We have a very simple use case where are python app sends SMS texts to on-call users when system alerts happen.  I have created in AWS API acccess id/keys, and a toll-free number through Pinpoint.</p>
  <p>My publish function looks like this:</p>
  <pre><code>        try:
              sns = boto3.client('sns',
                                 aws_access_key_id='&lt;our api id&gt;',
                                 aws_secret_access_key='&lt;our api key&gt;',
                                 endpoint_url='https://sns-fips.us-east-2.amazonaws.com',
                                 config=Config(region_name='us-east-2',
                                               proxies={'https': '&lt;our proxy&gt;'}))
              for o in users:
                  number = getattr(o,'number') if getattr(o,'number').startswith('+') else '+1' + getattr(o,'number').replace(&quot;-&quot;,&quot;&quot;).replace(&quot;(&quot;,&quot;&quot;).replace(&quot;)&quot;,&quot;&quot;)
                  print('Sending message ' + msg + ' to contact ' + getattr(o,'name') + ' at number ' + number)
                  result = sns.publish(PhoneNumber=number, Message=msg, MessageAttributes={'AWS.MM.SMS.OriginationNumber': {'DataType': 'String', 'StringValue': '+1&lt;our tollfree&gt;'}})
                  print(result)
          except Exception as ex:
              print(&quot;SMS API EXCEPTION: {0}&quot;.format(ex), flush=True)
  </code></pre>
  <p>If I don't include the MessageAttributes, the message sends just fine but gets the AWS assigned long number (which is going away soon).  If I go into my SNS dashboard and manually send a message using our tollfree, it goes through.  If change the toll free to something else, it doesn't go through.  So I know the association is there.</p>
  <p>I also tried changing the code to use the Boto pinpoint client and get the same behavior, no messaging occurs, no errors.</p>
  <p>Using the sns publish, my results come back with a status code of 200, a request id, 0 retry attempts, etc.  So it seems like the messages are being sent.  And I see nothing in the SMS delivery logs.</p>
  <p>Am I missing something here?  Thanks in advance for any pointers.</p>