diff --git a/moto/sns/models.py b/moto/sns/models.py
index bc80f9e41..6f5ee54ac 100644
--- a/moto/sns/models.py
+++ b/moto/sns/models.py
@@ -173,6 +173,7 @@ class SNSBackend(BaseBackend):
         self.platform_endpoints = {}
         self.region_name = region_name
         self.sms_attributes = {}
+        self.opt_out_numbers = ['+447420500600', '+447420505401', '+447632960543', '+447632960028', '+447700900149', '+447700900550', '+447700900545', '+447700900907']
 
     def reset(self):
         region_name = self.region_name
diff --git a/moto/sns/responses.py b/moto/sns/responses.py
index 93786e72e..5b1c34610 100644
--- a/moto/sns/responses.py
+++ b/moto/sns/responses.py
@@ -521,7 +521,19 @@ class SNSResponse(BaseResponse):
 
     def list_phone_numbers_opted_out(self):
         template = self.response_template(LIST_OPTOUT_TEMPLATE)
-        return template.render(opt_outs=['+447420500600', '+447420505401'])
+        return template.render(opt_outs=self.backend.opt_out_numbers)
+
+    def opt_in_phone_number(self):
+        number = self._get_param('phoneNumber')
+
+        try:
+            self.backend.opt_out_numbers.remove(number)
+        except ValueError:
+            pass
+
+        template = self.response_template(OPT_IN_NUMBER_TEMPLATE)
+        return template.render()
+
 
 
 CREATE_TOPIC_TEMPLATE = """<CreateTopicResponse xmlns="http://sns.amazonaws.com/doc/2010-03-31/">
@@ -876,3 +888,11 @@ LIST_OPTOUT_TEMPLATE = """<ListPhoneNumbersOptedOutResponse xmlns="http://sns.am
     <RequestId>985e196d-a237-51b6-b33a-4b5601276b38</RequestId>
   </ResponseMetadata>
 </ListPhoneNumbersOptedOutResponse>"""
+
+OPT_IN_NUMBER_TEMPLATE = """<OptInPhoneNumberResponse xmlns="http://sns.amazonaws.com/doc/2010-03-31/">
+  <OptInPhoneNumberResult/>
+  <ResponseMetadata>
+    <RequestId>4c61842c-0796-50ef-95ac-d610c0bc8cf8</RequestId>
+  </ResponseMetadata>
+</OptInPhoneNumberResponse>
+"""
diff --git a/tests/test_sns/test_sms_boto3.py b/tests/test_sns/test_sms_boto3.py
index a10f9d6dc..9fbc90ac8 100644
--- a/tests/test_sns/test_sms_boto3.py
+++ b/tests/test_sns/test_sms_boto3.py
@@ -67,5 +67,18 @@ def test_list_opted_out():
     response = conn.list_phone_numbers_opted_out()
 
     response.should.contain('phoneNumbers')
-    response['phoneNumbers'].should.contain('+447420500600')
-    response['phoneNumbers'].should.contain('+447420505401')
+    len(response['phoneNumbers']).should.be.greater_than(0)
+
+
+@mock_sns
+def test_opt_in():
+    conn = boto3.client('sns', region_name='us-east-1')
+    response = conn.list_phone_numbers_opted_out()
+    current_len = len(response['phoneNumbers'])
+    assert current_len > 0
+
+    conn.opt_in_phone_number(phoneNumber=response['phoneNumbers'][0])
+
+    response = conn.list_phone_numbers_opted_out()
+    len(response['phoneNumbers']).should.be.greater_than(0)
+    len(response['phoneNumbers']).should.be.lower_than(current_len)