To sign in to your extension from a phone that is different than the one you normally use, press the voicemail button on the phone.
It will ask you to enter the password of the extension currently on the phone. Ignore that, and press pound again. From there, it will ask you to enter your extension, and then your voicemail password. Once you are logged in, you can press 7, then 3, and then 1, and it will sign your extension to that phone.
Use the same process when you are back at your normal phone to change it back.