The first question is: why are you putting a conference call in a project schedule? If this is just a "status update" then I wouldn't even bother -- there's no deliverable and the absence of a resource will not prevent the task from completing.
OTOH, if this is a "sign off" call where there is a deliverable (documentation that a particular deliverable has been approved) then it should be there.
You have to be careful, too, about simply creating the task as Fixed duration and then assigning resources. If you create the task as Fixed Duration then the default duration and default work are 8hr. If you then come along and assign 24esources then each resource will get 20 minutes of work. Is that what you want?
Me? If I *had* to put it in then I'd create it, set the duration to 1 hr (or however long the meeting will take) and then I'd make certain that the "Type" was "Fixed Duration" and the "Effort Driven" flag was set to "No".
Now, I'd press Alt+F10 and assign the resources to the task. The duration will remain fixed at 1 hr ("Fixed Duration") and each resource will be assigned an hour ("Effort Driven" = "no") instead of the work (1hr) being divided up among the participants. It's also best to use the "Effort Driven = No" setting in case you will be adding or removing resources and you don't want to adjust the amount of work that each resource is doing during this meeting.
Note that the settings of "Type" and "Effort Driven" will vary from task to task depending on what you are trying to accomplish at the time you are updating it.