Conference Bridge URL

Our client would like to be able to have more than 3 people on a conference call. I have set up conference rooms for them, but some times the call starts in a 3-way and the boss would like to be able to continue to add more people on the fly. This is a big-time Hollywood movie director/producer and he doesn't have a lot of time and patience. I read that Cisco SPA 5xx phones have the "Conference Bridge URL" option. If you put the conference URL "number@domain.com", and then us the conf option on the phone, the calls will be transferred to that designated conference room and then you can continue the call and just keep adding people.

For the life of me, I have not been able to get this to work, however. I have tried just the number, the name, the name with the port number, the external name. I have checked the call log, and it does show that it is trying to transfer the call, but the hangup cause says "INCOMPATIBLE DESTINATION".

Any ideas folks?