Rather frustrating this one. I've seen this on two different customer deployments of CME so far. Both are on 2800 routers and in each situation attached to different devices (one customer the port was attached to a fax, one it was attached to a Cisco phone).
The FXS port itself is flagged as FXSLS_PARK under "show voice call summary". A quick shut/no shut the interface and it comes back to a EFXS_OFFHOOK state. Then eventually it times out and drops to FXSLS_PARK state again. This would be normal behavior if the device/s actually was offhook.
I know a reboot fixes the issue. But it frustrates the customer to find this out (especially after they approach two other parties before us) and also frustrates myself not being able to offer more advice than simply "it's locked up mate, restart".
I figure the port is mis-interpreting the signal state on the line but I've not figured out why/how to fix it.