[Freeswitch-users] Extract variables from 300 redirect

Hi Guys I am wondering if its possible to extract a custom variable from a 300 Redirect response from a billing system? Effectively I am trying to add a custom variable to the 300 message to give a maximum call length to then use in the dial plan for a Scheduled Hangup. Any pointers of how to do this or the magic words to search confluence/google with? Regards