SIP clients keep on ringing

Hi everybody.
I’m facing a problem while dialing multiple SIP extensions.
Here’s what happens.
When someone is calling my company number, Asterisk dials all SIP clients and all the connected ones correctly ring. But after a SIP client picks up the call, other clients keep on ringing. If I pick up the call with one of these clients, actually I don’t here anything and that’s correct, as someone else already pickup.
Here’s my system configuration:
Digium TE110P on Fedora Core 3.0 (kernel 2.6.9), Zaptel 1.0.9, libpri 1.0.9, Asterisk 1.0.9.
As client I’m using eyeBeam and here’s my configuration files:

[zapata.conf]

[channels]
language=it
usecallingpres=yes
cancallforward=yes
signalling=pri_cpe
switchtype=euroisdn
pridialplan=unknown
echocancel=yes
echocancelwhenbridged=yes
echotraining=400
callerid=asreceived
group=1
context=default
language=it
musiconhold=default
channel => 1-15,17-31

[part of extensions.conf]

[globals]
ALLSIPS=SIP/100&SIP/101&SIP/102

[default]
exten => _123456789,1,Dial(${ALLSIPS},15,Ttr)
exten => _123456789,2,Goto(_123456789-${DIALSTATUS},1)

exten => _123456789-NOANSWER,1,Goto(_123456789-NO,1)
exten => _123456789-BUSY,1,Goto(_123456789-NO,1)
exten => _123456789-CHANUNAVAIL,1,Goto(_123456789-NO,1)
exten => _123456789-CONGESTION,1,Goto(_123456789-NO,1)

exten => _123456789-NO,1,Playback(unavailable)
exten => _123456789-NO,2,Wait(2)
exten => _123456789-NO,3,Voicemail(s123123)
exten => _123456789-NO,4,Hangup

Thanks,
fangi