Cannot get incoming calls on my Mitel 5212 IP phone. When my colleagues phone internally, they get the "NO DISTB" message.

possible dropped out the LAN driver
good bet if you are using win xp or win 7 OS

Need to login to. The phone system and change it back.

Call your office admin or tech that programs the system at your site to allow access.

How can you pick up an incoming call from another handset?

Log into your mailbox. Press U(8) for User Options. Press G(4) for Greeting. Listen to the prompts.

I have exchanged the telephone on my desk but cannot get a ring tone or flashing lights for incoming calls?

It sounds like your phone is in do-not-disturb. Press the blue key. Press the no (# key) until you see DND, then press Yes (* key). Press Turn off (0 key).

this is a change your system administrator will need to handle, (or your phone tech). There is an option in the under "system options" called internal ring cadence for external callers; this is where you can have the system do a double ring for External and a single ring for internal or vice versa.

This can be handled by the DND feature in your admin settings... Do not disturb will transfer your calls direct to voicemail without ringing the handset.

There are a number of Headset that will plug directy into the back of your 5212 phone. (I would suggest Googling for results) As for Monitoring an agent the Mitel ICP has the capability to allow a supervisor to dial a code and mointor the agent while on a call. Contact you Mitel Vendor and I'm sure they would be happy to make the program chage for you. (This is a class of service and Feature acces code change)