Hi Jeevaka,

> +	unsigned char addnl;

Lets try to be consistent with other parts of stk.c.
Use unsigned char no_cause[] = { 0x00 };

>  
>  	if (stk->pending_cmd->send_ussd.alpha_id &&
>  			stk->pending_cmd->send_ussd.alpha_id[0])
> @@ -1661,7 +1662,14 @@ static void send_ussd_callback(int error, int dcs, const unsigned char *msg,
>  		break;
>  
>  	default:
> -		send_simple_response(stk, STK_RESULT_TYPE_USSD_RETURN_ERROR);
> +		rsp.result.type = STK_RESULT_TYPE_USSD_RETURN_ERROR;
> +		addnl = 0; /* No specific cause can be given */
> +		rsp.result.additional = &addnl;
> +		rsp.result.additional_len = 1;
> +

Then rsp.result.additional_len = sizeof(no_cause);
rsp.result.additional = no_cause

> +		if (stk_respond(stk, &rsp, stk_command_cb))
> +			stk_command_cb(&failure, stk);
> +
>  		break;
>  	}
>  }

Otherwise looks good.

Regards,
-Denis