From Michael Gueterman:

If you really want all the capabilities of the console, then what you need to do is enable "remote console" access by issuing the "ER" (enable remote) command from the maintenance mode prompt (hit <cntrl><B> from LDEV 20). You may also need to configure the baud rate, parity, etc., which can be accomplished with the "CO" command. Following that, plug your modem into the remote access port (in normal mode it is usually LDEV 21), and again at the CM> prompt, issue the "UL" (unlock remote) command (just in case). When the remote is enabled and unlocked (which can be verified from LDEV 20 when at the CM> prompt by checking if REMOTE: enabled is displayed in the status line at the bottom of the display), you can become the physical console by hitting the <BREAK> key. At that point, you are LDEV 20, not 21. If someone at the actual console hits the <BREAK> key, they get the console back. When you are not the console, all output is displayed back to you so you can monitor what the other console is doing without being able to do anything yourself until you hit <BREAK>.

A couple of caveats:

Make sure you use a proper cable that supports full modem signals. That way, if you lose the connection (even if you had LDEV 20), the console reverts back.

There is an option when you enable the remote access port to place a password on that port. Do it! Otherwise, anyone who happens to get that phone number can do some less than grand things to your system when the remote is enabled. Note that this may effect Predictive Support’s dial out capability.