This is possible if an outbound API call is made from the client's end. As a part of the API OB call the client would 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 "StatusCallback" parameter, and provide the status of the call and other parameters(mentioned above). The client would need to capture the "Call Status" and if the status says "completed", then SMS API code should be used to send out an SMS to their Customer.

    Need a techie for this integration?
    Check our Phone a Friend App