When the CallBack function is activated, the GSM gateway refuses to receive this incoming call (the call is only ringing) for a pre-defined time.In case the calling party terminates the call before the timeout, the GSM gateway starts calling to a defined extension.
When the extension hangs up the call, the gateway starts calling to the GSM.
For e.g. when the mobile user wishes to place a call, they first call the PABX and hang up, after which the PABX returns the call and allows them to place a call at much lower costs. This is especially useful when user is roaming, as incoming call costs are lower than outgoing
