asterisk users - Oct 2008 - Unknown call every 30 minutes on the dot.

Here's some freaky stuff coming from Areski CDR tool:

101.  2008-10-13 03:41:23 DAHDI/1... 0000000 "unknown" <0000000>
BackGround
silence/5 s
ANSWERED 00:20

 102.  2008-10-13 03:11:30 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21

 103.  2008-10-13 02:41:23 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21

 104.  2008-10-13 02:11:22 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21

 105.  2008-10-13 01:41:21 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21

 106.  2008-10-13 01:11:21 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21

 107.  2008-10-13 00:41:29 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21

 108.  2008-10-13 00:11:21 DAHDI/1... 0000000 "unknown"
<0000000> BackGround
silence/5 s
ANSWERED 00:21


When Asterisk see an incoming call without a caller ID, it sets it to
"unknown" and 0000000. As you can see from the list above, it happens
every
30 minutes almost to the second. It is still happening right now, unless
that line is in use, in which case it'll try again 30 minutes later.

I did notice this in the /var/log/asterisk/full log:

[Oct 13 03:11:30] NOTICE[4243] chan_dahdi.c: Got event 17 (Polarity
Reversal)...
[Oct 13 03:11:38] WARNING[4243] chan_dahdi.c: CallerID returned with error
on channel 'DAHDI/1-1'

Normally, it says:

[Oct 12 13:23:59] DEBUG[30124] chan_dahdi.c: Ignore switch to REVERSED
Polarity on channel 1, state 4
[Oct 12 13:23:59] DEBUG[30124] chan_dahdi.c: Ignoring Polarity switch to
IDLE on channel 1, state 4
[Oct 12 13:23:59] DEBUG[30124] chan_dahdi.c: Polarity Reversal event occured
- DEBUG 2: channel 1, state 4, pol= 0, aonp= 0, honp= 0, pdelay= 600, tv=
-233440198

Any clues?

TDM410P with 2 FXO ports and EC module. Running Fedora Core 9 in init:3 with
USB disabled (to prevent IRQ conflicts).
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