Long distance calls drop

Hello, can anyone advise on what can cause the following.

We are running Asterisk 1.4.25 and Polycom phones (IP 430, updated to the latest firmware).
Asterisk server is on CentOS 5.1 (kernel 2.6.18-128.1.10.el5), has Digium card installed.

From time to time, incoming calls drop. It looks like it’s just long distance calls coming on the “ground” (Bell) lines, but we are not 100% sure.

Also, it looks like they drop after 12-20 minutes.

I’ve enabled debugging but don’t see anything unusual. When the call is dropped, there is the following line:
– Hungup 'DAHDI/2-1

which will be the same if I just hang up by myself.

The server has been running for a few month already but this issue has started happening just a few weeks ago.

What can be possible causes for this kind of problem? Where should I dig to be able to find the problem and fix it?

Any thoughts/advices will be appreciated.

If you are using analogue lines, you should be able to get lower level debugging information from either chan_dahdi or the device driver itself, which show the exact line states (e.g. line reversals, loss of battery, etc.). I don’t use dahdi for such lines, but I have seen other people producing such logs here.