add sms messaging support

lib/session/sms-call-session.js

@@ -0,0 +1,22 @@
+const CallSession = require('./call-session');
+
+/**
+ * @classdesc Subclass of CallSession.  Represents a CallSession
+ * that is established for the purpose of sending an outbound SMS
+ * @extends CallSession
+
+ */
+class SmsCallSession extends CallSession {
+  constructor({logger, application, srf, tasks, callInfo}) {
+    super({
+      logger,
+      application,
+      srf,
+      tasks,
+      callInfo
+    });
+  }
+
+}
+
+module.exports = SmsCallSession;