User got used to pressing one button to activate the service, thus "capturing" the line, so when the called party hangs, a call is placed between them.

While this is quite easy to implement in Asterisk, and even "hacks" to achieve this in the community, the result is far from optimal. Current developments simulate that callback by placing the callback reversely, starting at the callee phone, and ending at the caller one. This behaviour is inverse to what the user has been using for a long time, and in the short term can produce multiple problems.

At EuropeSIP labs, we have developed a full implementation of the former behaviour through a daemon which controls the process, in which the user at the caller side is who starts the callback, and not the reverse. And we have added a great value, by making it multi-location aware, between connected pbx, even geographically scattered, so a user from ie. Madrid, can activate the service against a Barcelona or New York user.

The callback scheme is as follows:

1. Calling party, after calling busy destination, listens to a hearing pointing the possibily to activate the callback by pressing 6.

2. When called party hangs, our script gets an event, and searches through the database, checking if there is a callback service activated.

3. If affirmative and being under the callback expiry time, the script then checks if the parties are available. Should any of them be busy, the callback is postponed.

4. If either of them are idle, then the callback is placed with a caller id as follows:
  
   Callback from ext1. to ext2.
  
   where ext1 is the calling party and ext2 is the called one.

5. Each 10 minutes, a broom process is launched in order to try placing, after all the above tests passed, the postponed callbacks.

The callback expiry time and the broom proccess timer are subject to configuration.

The code and the setup guide are available freely of charge under our registerd users' download area.