This is possible if an outbound API call is made from your end. As a part of the API outbound call, you would need to pass on an additional parameter "StatusCallback".
For example, "StatusCallback" = "http://example.com/exotel/obcalls" in addition to the standard call parameters like "From" & "To".
After the outbound call, we will make an HTTP call to that URL specified on the "StatusCallback" parameter, and provide the status of the call and other parameters (mentioned above). You would need to capture the "Call Status" and if the status says "completed", then an SMS API code should be used to send out an SMS to their customer. Please refer to https://developer.exotel.com/api/sms.
Note: The SMS API needs to be configured at your end once you have received the call status and this will be triggered separately. SMS charges will be applicable as per your Bill Plan.
Also check out: Sending an SMS