@thwr here is what the info page had to offer:
Problem Description:
If you sometimes see this error on a zaptel card, this might be caused by a network card or a harddisk taking an interrupt for too long. This will cause the calls to be dropped.
This might be caused by:
Too much I/O activity
bad hardware
bad drivers
If you see this error non stop scrolling on the asterisk CLI, you probably made a configuration error,
Doublecheck your coding / framing specified in zaptel.conf.
The below settings worked.
Possible solutions:
If you are using an intel 100mbit network card, try replacing the standard drivers with the drivers from the intel website. The drivers included in the kernel have some latency problems
replace on board nics with some add-in cards
Try looking if you have any shared interrupts, and try playing with disabling onboard network cards and plugging in new ones.
See also here
Try replacing SATA controllers with IDE controllers and tweak the hdparm settings. see this tutorial
Stop using the harddisk on the server as much. (get rid of updatedb lines in the cron jobs).
upgrade to a dual cpu system, more cpu's = an extra cpu to take an interrupt if the first one is busy with an interrupt that is waiting too long.
If you are using a dual port or quad port PRI digium card, you could try to do the hdlc in hardware instead of in software.
enable ACPI in the kernel config
enable your chipset in the kernel config