UK ISDN : Can only dial out on alternate channels

I’m using a B410P, connected to 2 UK (BT) ISDN lines (4 voice channels), running Asterisk 1.6.2.9, DAHDI 2.3.0.1 and LibPri 1.4.11.3, running on Ubuntu 10.04.

After getting the system up and running, and getting the first inbound and outbound calls to work, further experimentation has revealed that I can only dial out on channels 1 and 4. Channels 2 and 5 don’t appear to work.

I’ve set $TRUNK=r1 (i.e. increasing round robin) so that I can check each of the 4 channels in turn, and every time a call goes out on channels 2 or 5, the following is output on the asterisk console:

    -- Executing [9##########@DLPN_Standard:1] Dial("SIP/502-00000012", "DAHDI/r1/##########") in new stack
    -- Requested transfer capability: 0x00 - SPEECH
    -- Called r1/9##########
    -- DAHDI/2-1 is proceeding passing it to SIP/502-00000012
    -- DAHDI/2-1 is ringing
    -- DAHDI/2-1 answered SIP/502-00000012

…and as soon as I hang up, the following message appears

    -- Hungup 'DAHDI/2-1'
[trunkgroups]

[channels]
language=en
switchtype=euroisdn
usecallerid=yes
cidsignalling=v23
callwaiting=no
usecallingpres=yes
callwaitingcallerid=yes
threewaycalling=no
transfer=no
canpark=no
cancallforward=no
callreturn=yes
echocancel=yes
echocancelwhenbridged=no
pridialplan=unknown
prilocaldialplan=dynamic
context=incoming
group=1
callgroup=1
pickupgroup=1
signalling=bri_cpe
channel => 1,2,4,5
root@voip2:/usr/src# cat /etc/dahdi/system.conf | grep -v ^#
span= 1,1,0,ccs,ami
bchan = 1,2
hardhdlc = 3
span=2,2,0,ccs,ami
bchan = 4,5
hardhdlc = 6
loadzone = uk
defaultzone=uk
voip2*CLI> dahdi show channels
   Chan Extension  Context         Language   MOH Interpret        Blocked    State
 pseudo            default                    default                         In Service
      1            incoming        en         default                         In Service
      2            incoming        en         default                         In Service
      4            incoming        en         default                         In Service
      5            incoming        en         default                         In Service
voip2*CLI> dahdi show status
Description                              Alarms  IRQ    bpviol CRC4   Fra Codi Options  LBO
B4XXP (PCI) Card 0 Span 1                OK      0      0      0      CCS AMI  YEL      0 db (CSU)/0-133 feet (DSX-1)
B4XXP (PCI) Card 0 Span 2                OK      0      0      0      CCS AMI  YEL      0 db (CSU)/0-133 feet (DSX-1)
B4XXP (PCI) Card 0 Span 3                RED     0      0      0      CAS Unk  YEL      0 db (CSU)/0-133 feet (DSX-1)
B4XXP (PCI) Card 0 Span 4                RED     0      0      0      CAS Unk  YEL      0 db (CSU)/0-133 feet (DSX-1)

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