asterisk users - Mar 2006 - Queue and busy/congested ZAP channels

Hi,

I'm having a problem with the queue behaviour in my place:

I have two ISDN channels to the outside (Zap/1) and two channels two a
Siemens Gigaset (Zap/4). I also use a SIP gateway to call outside and
have a couple of IP phones around as well (SIP).

The Gigaset has about 5 phones connected to it (+base station). Whenever
two people are using those, I always am blocking two internal channels,
so users who call in from the outside always get a busy or the
voicemail.

I configured a queue to take all calls to our central number, so people
will get to a phone as soon as possible. However, when there are only
agents logged on from the Zap/4 line, and both channels are used, the
caller gets transferred to the voicemail of one of the agents instead of
beeing put into the queue. 

Here is some console output on those events:

    -- Accepting voice call from '1797808366' to '12298890' on
channel
0/1, span 1
    -- Executing Queue("Zap/1-1", "reception") in new stack
    -- Started music on hold, class 'default', on channel
'Zap/1-1'
    -- outgoing agentcall, to agent '1003', on
'Local/1003@public-phones-548f,1'
    -- Called Agent/1003
    -- outgoing agentcall, to agent '1000', on
'Local/1000@public-phones-c4bb,1'
    -- Called Agent/1000
    -- Executing Macro("Local/1003@public-phones-548f,2",
"call-user|
1003") in new stack
    -- Executing Dial("Local/1003@public-phones-548f,2",
"Zap/g4/1003&Sip/S1003&Sip/1003|45|tr") in new stack
Mar 21 09:57:38 NOTICE[4354]: app_dial.c:1030 dial_exec_full: Unable to
create channel of type 'Zap' (cause 34 - Circu
it/channel congestion)
    -- Executing Macro("Local/1000@public-phones-c4bb,2",
"call-user|
1000") in new stack
    -- Executing Dial("Local/1000@public-phones-c4bb,2",
"Zap/g4/1000&Sip/S1000&Sip/1000|45|tr") in new stack
Mar 21 09:57:38 NOTICE[4356]: app_dial.c:1030 dial_exec_full: Unable to
create channel of type 'Zap' (cause 34 - Circu
it/channel congestion)
Mar 21 09:57:38 WARNING[4354]: chan_sip.c:1973 create_addr: No such
host: S1003
Mar 21 09:57:38 NOTICE[4354]: app_dial.c:1030 dial_exec_full: Unable to
create channel of type 'Sip' (cause 3 - No rou
te to destination)
Mar 21 09:57:38 WARNING[4356]: chan_sip.c:1973 create_addr: No such
host: S1000
Mar 21 09:57:38 NOTICE[4356]: app_dial.c:1030 dial_exec_full: Unable to
create channel of type 'Sip' (cause 3 - No rou
te to destination)
Mar 21 09:57:38 WARNING[4354]: chan_sip.c:1973 create_addr: No such
host: 1003
Mar 21 09:57:38 NOTICE[4354]: app_dial.c:1030 dial_exec_full: Unable to
create channel of type 'Sip' (cause 3 - No rou
te to destination)
  == Everyone is busy/congested at this time (3:0/1/2)
    -- Executing VoiceMail("Local/1003@public-phones-548f,2",
"u1003")
in new stack
    -- Playing 'vm-theperson' (language 'en')

Any ideas why it goes to voicemail instead of keeping the caller in the
queue?

Thanks,
Christian