I you have an Exotel Coupon, you can redeem it by doing the following:
1) Go to http://my.exotel.in/pay
2) Choose the Discount/Coupon Code option from the accordion
3) Enter your coupon code and click redeem
Once you've done that-
Prepaid accounts: You'll see that your coupon amount has been added to your Exotel balance
Postpaid accounts: You will see that in your forthcoming bill, a discount worth the amount on the coupon is given to you