diff --git a/lib/rack/twilio_webhook_authentication.rb b/lib/rack/twilio_webhook_authentication.rb
index 364decbd7..2337fe02f 100644
--- a/lib/rack/twilio_webhook_authentication.rb
+++ b/lib/rack/twilio_webhook_authentication.rb
@@ -17,18 +17,20 @@ module Rack
   # doesn't validate then the middleware responds immediately with a 403 status.
 
   class TwilioWebhookAuthentication
-    def initialize(app, auth_token, *paths)
+    def initialize(app, auth_token, *paths, &auth_token_lookup)
       @app = app
       @auth_token = auth_token
+      define_singleton_method(:get_auth_token, auth_token_lookup) if block_given?
       @path_regex = Regexp.union(paths)
     end
 
     def call(env)
       return @app.call(env) unless env["PATH_INFO"].match(@path_regex)
-      validator = Twilio::Util::RequestValidator.new(@auth_token)
       request = Rack::Request.new(env)
       original_url = request.url
       params = request.post? ? request.POST : {}
+      auth_token = @auth_token || get_auth_token(params['AccountSid'])
+      validator = Twilio::Util::RequestValidator.new(auth_token)
       signature = env['HTTP_X_TWILIO_SIGNATURE'] || ""
       if validator.validate(original_url, params, signature)
         @app.call(env)
@@ -41,4 +43,5 @@ def call(env)
       end
     end
   end
+
 end
diff --git a/spec/rack/twilio_webhook_authentication_spec.rb b/spec/rack/twilio_webhook_authentication_spec.rb
index ed1ee4a6b..63f53a6ee 100644
--- a/spec/rack/twilio_webhook_authentication_spec.rb
+++ b/spec/rack/twilio_webhook_authentication_spec.rb
@@ -18,6 +18,28 @@
         Rack::TwilioWebhookAuthentication.new(@app, 'ABC', /\/voice/, /\/sms/)
       }.not_to raise_error
     end
+
+    it 'should initialize with an app, dynamic token and paths' do
+      expect {
+        Rack::TwilioWebhookAuthentication.new(@app, nil, /\/voice/, /\/sms/)
+      }.not_to raise_error
+    end
+  end
+
+  describe 'calling against one path with dynamic auth token' do
+    it 'should allow a request through if it validates' do
+      auth_token = 'qwerty'
+      account_sid = 12345
+      expect_any_instance_of(Rack::Request).to receive(:post?).and_return(true)
+      expect_any_instance_of(Rack::Request).to receive(:POST).and_return({'AccountSid' => account_sid})
+      @middleware = Rack::TwilioWebhookAuthentication.new(@app, nil, /\/voice/) { |asid| auth_token}
+      request_validator = double('RequestValidator')
+      expect(Twilio::Util::RequestValidator).to receive(:new).with(auth_token).and_return(request_validator)
+      expect(request_validator).to receive(:validate).and_return(true)
+      request = Rack::MockRequest.env_for('/voice')
+      status, headers, body = @middleware.call(request)
+      expect(status).to be(200)
+    end
   end
 
   describe 'calling against one path' do