To do an intercom (a two way call that automatically answers on speakerphone at the recipient's phone) to another extension, you can dial 2 + the extension you want to call. For instance, if you want to intercom Bob at extension 101, you would dial 2 + 101. A warning beep will sound on Bob's phone before he hears you via the intercom.
You might also have a One Touch button on your phone that automatically intercoms a person when pressed. You can identify this button by either the description on the phone's display, or the intercom icon as highlighted below.