Outgoing calls fail to disconnect

I have a small FreePBX system with a dozen extensions and outbound calls from one extension sometimes fail to disconnect once the call has been completed. The only resolution so far has been to reboot the handset. I have changed the handset, changes the RJ-45 connection but the problem remains, only with this one extension. I have FreePBX ver running on Ubuntu 9 with no other applications running on this server.