Or

The conferance host always uses the same chairman and participant codes and publishes both in the eMail invite.

Which means anyone else he's used it with knows it, which leads to fun, I've been on calls where a participant dials in as the chairman forcing the chairman to dial in as a particpant, he never commented on it.

Other gems are that when someone leaves, they just transfer the numbers to someone else. When a senior sales person leaves, they don't change all the codes in the sales team so it is not unknown for sales people to dial into their old companies to see what's going on.

And usualy no one challenges blanck call identifiers.