You can set up auto response messages on incoming SMS using SMS on Workflow feature. With SMS on workflow you can define actions for workflow rules & send auto replies to incoming queries.  For e.g.: if you receive response as Yes from your customer send auto response as "Thank you for showing interest"

For this,  you need to understand how to use “SMS On Workflow” feature. You will have to create a workflow rule on Incoming SMS object and then set up the workflow for sending auto message. 

Let me guide you step by step on how to setup auto response SMS-:

Step 1-: Create SMS Template for auto response with template type as "Incoming SMS". 

          You can refer below link if you need any help in setting up template-:
Step 2-: Generate "Configuration Name" with SMS On Workflow addon:
          You need to generate "Configuration Name" with the help of "SMS On Workflow" addon. For the same, goto "SMS On Workflow" tab 
          of "SMS On Workflow" app and click on "New SMS Workflow".
          Then select the lookup object as incoming SMS i.e. smagicinteract__Incoming_SMS__c 
           Recipient Type as "Mobile Phone field" and select the Mobile number field. then select the sender Id and name field as 
           per your requirement and also select the sms template which you created in step 1.
          Click on "Create" to generate "Configuration Name" and then copy the Configuration Name generated 
           from "Copy Configuration Name" section.

Step 3: Setup Workflow rule on incoming SMS object
           Now you can setup workflow rule on incoming SMS object and set the required rule & evaluation criteria. In this case, we are checking if the incoming response i.e. text contains "Yes" or not.
Step 4-: Set an action as "Create Task" & Paste the Configuration Name generated in step 2 in this Task comment section.
Step 5-: Activate the workflow rule
         Once you activate the workflow rule, whenever new incoming response is received with keyword as Yes, an auto response with 
         text "Congratulations...You've just made your most important decision! We look forward to see you .." will be delivered to the customer who has 
          responded to you.