Looking at the log file I found the following error so I had to restart sip server:
 
May  2 12:00:45 debian kernel: Zapata Telephony Interface Registered on major 196
May  2 12:00:45 debian kernel: No ISA tormenta card found at d0000
May  2 12:00:45 debian kernel: Zapata Telephony Interface Unloaded
May  2 12:00:45 debian insmod: /lib/modules/2.4.20-8smp/misc/torisa.o: init_module: Input/output error
May  2 12:00:45 debian insmod: Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters.       You may find more information in syslog or the output from dmesg
May  2 12:00:45 debian insmod: /lib/modules/2.4.20-8smp/misc/torisa.o: insmod char-major-196 failed
 
Actually I don't know what Zapata Telephony Interface is, however I guess the problem is loading the torisa module ??? any help please ?
 
Giuseppe