clicking on handset

[jpbody]

We have a fortivoice fve-20e2. We have 4 fon-670i phones 2 in the office and 2 out at remote locations using vpn to connect in.  For some reason the 2 phones in the office have a clicking sound on the handsets.  You can use the speakerphone on them and it and you don't hear the clicking but if you pick up the hand set even during the dial tone there is a clicking noise.  The 2 at the remote sites that connect in with vpn do not have this at all.  I have taken one of the phones from the office to one of the remote locations and hooked it up and it still has the clicking in the handset.  If I leave it unplugged for a few weeks and then plug it in and try to use it the clicking will be gone but within a week or so it will start again.  It starts off slow then will over time will start to speed up not while you are using it just when it starts it will be slow then the next day you pick it up and it will be quicker and then the next even quicker till you her a few clicks a second.  Has anybody ever had this problem and if so any suggestions to clear it up.

Thank you