asterisk users - Jan 2006 - SIP phones can't pick up incoming call on analog trunk - signalling problem?

A very good day to you all,

We can't get the phones to pick up on an incoming call on analog trunks.  
We're using the digium products in the box, with snom phones internally.

This is the output from the asterisk console:
linux*CLI> zap show channels
   Chan Extension  Context         Language   MusicOnHold
 pseudo pstn-incoming   en         default
      1            pstn-incoming   en         default
      2            pstn-incoming   en         default
      3            pstn-incoming   en         default
      4            pstn-incoming   en         default
    -- Starting simple switch on 'Zap/1-1'
Jan 13 13:32:13 WARNING[25905]: chan_zap.c:6105 ss_thread: CallerID returned 
with error on channel 'Zap/1-1'
    -- Executing Dial("Zap/1-1", "SIP/24|15") in new stack
    -- Called 24
    -- SIP/24-406f is ringing
    -- SIP/24-406f is ringing
    -- SIP/24-406f is ringing
    -- Starting simple switch on 'Zap/4-1'
Jan 13 13:32:16 NOTICE[25908]: chan_zap.c:6031 ss_thread: Got event 18 (Ring 
Begin)...
    -- SIP/24-406f is ringing
Jan 13 13:32:18 NOTICE[25908]: chan_zap.c:6031 ss_thread: Got event 2 
(Ring/Answered)...
    -- Executing Dial("Zap/4-1", "SIP/24|15") in new stack
    -- Called 24
    -- SIP/24-c8de is ringing
    -- SIP/24-c8de is ringing
  == Spawn extension (pstn-incoming, s, 1) exited non-zero on 'Zap/1-1'
    -- Hungup 'Zap/1-1'
    -- SIP/24-c8de is ringing
Jan 13 13:32:21 WARNING[25908]: chan_zap.c:3904 zt_handle_event: Ring/Off-hook 
in strange state 6 on channel 4
    -- SIP/24-c8de is ringing
Jan 13 13:32:24 WARNING[25908]: chan_zap.c:3904 zt_handle_event: Ring/Off-hook 
in strange state 6 on channel 4
    -- SIP/24-c8de is ringing
  == Spawn extension (pstn-incoming, s, 1) exited non-zero on 'Zap/4-1'
    -- Hungup 'Zap/4-1'
    -- Starting simple switch on 'Zap/4-1'
Jan 13 13:32:31 NOTICE[25916]: chan_zap.c:6031 ss_thread: Got event 18 (Ring 
Begin)...
Jan 13 13:32:33 NOTICE[25916]: chan_zap.c:6031 ss_thread: Got event 2 
(Ring/Answered)...
    -- Executing Dial("Zap/4-1", "SIP/24|15") in new stack
    -- Called 24
    -- SIP/24-9f09 is ringing
    -- SIP/24-9f09 is ringing
    -- SIP/24-9f09 is ringing
    -- SIP/23-b60e answered Zap/4-1
Jan 13 13:32:36 WARNING[25916]: chan_zap.c:3904 zt_handle_event: Ring/Off-hook 
in strange state 6 on channel 4
  == Spawn extension (pstn-incoming, s, 1) exited non-zero on 'Zap/4-1'
    -- Hungup 'Zap/4-1'


& zapata.conf

[channels]
language=en
toneduration=100
context=pstn-incoming
signalling=fxs_ks
usecallerid=yes
hidecallerid=no
usecallingpres=yes
threewaycalling=yes
transfer=yes
canpark=yes
callreturn=no
echocancel=yes
echocancelwhenbridged=yes
rxgain=0.0
txgain=0.0
group=1
callgroup=1
pickupgroup=1
immediate=no
flash=100
transfertobusy=yes
busydetect=yes
busycount=4
busypattern=400,400
callprogress=yes
progzone=au
ringtimeout=6000
faxdetect=incoming
musiconhold=default
channel=>1 ;X100P
channel=>2 ;X100P
channel=>3 ;X100P
channel=>4 ;X100P

& zaptel.conf

fxsks=1-4
loadzone = au
defaultzone=au

& extensions.conf
...relevant part...
[pstn-incoming]
exten => s,1,Dial(SIP/24|15)
exten =>
s,n,Dial(SIP/21&SIP/22&SIP/23&SIP/24&SIP/25&SIP/26&SIP/27|120)


My experience with asterisk isn't with analog signalling and it has been a 
long time since I looked at zapata.conf with a keen eye.  I've never not
been
able to pick up a call so I'm a little baffled and that's why i'm
presuming
it's something to do with signalling.

Thanks