Hi Roberto,
Unfortunately this is not possible with regular meetings. In CWMS 2.5 you have a blast dial meetings, that have this call back option that might be something you are interested in, but I am not sure if it is exactly what you are looking for. Here is the information about blast dial.
Blast Dial
Blast dial provides a special type of meeting that can dial out to a pre-determined list of participant in seconds. It is a reservation-less, always available meeting that is designed to be launched by a single phone call to a dedicated telephone number. The CWMS system then dials out to all defined participants and joins them to a WebEx meeting. Intended for emergency response and similar situations, the CWMS administrator defines meeting attributes including security parameters, group membership, and the audio greetings that participants will hear.
If you want to find out more about this feature and usage, you can take a look at here links:
http://www.cisco.com/c/en/us/td/docs/collaboration/CWMS/2_5/Administration_Guide/Administration_Guide/Administration_Guide_chapter_01111.html#concept_C2EBCA827B624073B385437E81EA9E78
http://www.cisco.com/c/en/us/td/docs/collaboration/CWMS/2_5/User_Guide/User_Guide/User_Guide_chapter_01100.html
I hope this helps.
-Dejan