Under service parameters (System->service Parameters->Active call manager-> Cisco Call Manager) there is an option for Maximum Ad Hoc Conference The default value is 4, The other thing that needs to be looked at is the Maximum number of calls that DN is allowed. To see this click on Device, Phone. Search for your phone click when on the phone page clikc on the DN left collumn and then go to the bottom of the page. There is max number of calls and a busy trigger. The Max number of call must be set higher than the busy trigger.