====== callback ======

  * Reques String:
<code> EVENT=BackCall&orgidentity=orgidentity&exten=exten&targetdn=targetdn&callerid=callerid&user=user&password=password&pwdtype=pwdtype&userdata=userdata</code>
  * Description:
Call caller first, if answered, system call callee
  * Parameters:

^ Params      ^ Type       ^ Description          ^
|EVENT|String|BackCall|
|orgidentity|String|team identity string|
|exten|String|caller number|
|targetdn|String|callee number|
|callerid|String|callerId number, trunk callerId will be used if not set|
|user|String|username|
|password|String|password|
|pwdtype|String|password type: plaintext/md5|
|userdata|String|userdata in CDR|

  * Return Format：

<code>
|Retuen|code|Retuen|message
</code>

  * Type of the Return Value: String
  * Return values：

^ Value     ^ Description       ^
|<code>|Retuen|1|Retuen|message</code>|<code>|Retuen|succeed|Retuen|message</code>|
|<code>|Retuen|2|Retuen|message</code>|<code>|Retuen|failed|Retuen|message</code>|