Have a customer with an INT 5000 (2.1.0.13) and 550.8520 phones and Plantronic headsets (M12 and M22 bases).
One extension has a regular tick, almost like a heart beat, on the headset while idle or on a call. The tick matches the flash of the time colon on the display when idle.
Moved base to another extension and ticking followed. Replaced base and ticking went away ...for 24 hours. When it came back it was a double tick. Tried a different physical phone, still got the ticking. Switching out headset base with new one makes the ticking go away.
Changing out bases daily seems like a bad idea. Ideas?

If it follows the base, it sounds as though the base is bad or it's not compatible with the phone. The 8520 does not need an external amplifier for a headset. The clicking noise may be caused by a signal that is too hot for the base. Go to the Plantronics web site. They have compatibility charts available.