asterisk users - May 2009 - What happened here when transfering a call ? Circuit-busy ???

I call the firm from my portable at home (zoiper softphone). I have
internal extension 60, and I call the internal SIP-client 10 at the firm
via an IAX-connection over internet.

My colleague at phone 10 answers my call. I ask him to transfer me with
my colleague at extension 50. He then presses "transfer" on the
grandstream GXP2020 (I get music) and dials the number 50. Phone 50
rings... and I loose connection...

It is absolutely sure that my colleague's phone at extension 50 was not
occupied. He was just awaiting my call...

This is wat the CLI says (from log-files) :

[May 15 16:51:41] VERBOSE[2743] logger.c:     -- Executing [10 at intern:1]
Dial("IAX2/jonaskellens-10870", "SIP/10|60|tT|kK") in new
stack
[May 15 16:51:41] VERBOSE[2743] logger.c:     -- Called 10
[May 15 16:51:41] VERBOSE[2743] logger.c:     -- SIP/10-09ac3e10 is
ringing
[May 15 16:51:43] VERBOSE[2743] logger.c:     -- SIP/10-09ac3e10
answered IAX2/jonaskellens-10870
[May 15 16:54:08] VERBOSE[2743] logger.c:     -- Started music on hold,
class 'default', on IAX2/jonaskellens-10870
[May 15 16:54:14] VERBOSE[2743] logger.c:     -- Stopped music on hold
on IAX2/jonaskellens-10870
[May 15 16:54:14] VERBOSE[2743] logger.c:   == Spawn extension (intern,
50, 0) exited non-zero on 'IAX2/jonaskellens-10870'
[May 15 16:54:14] VERBOSE[2743] logger.c:     -- Executing [50 at intern:1]
Dial("IAX2/jonaskellens-10870", "SIP/50|60|tT|kK") in new
stack
[May 15 16:54:14] VERBOSE[2743] logger.c:     -- Called 50
[May 15 16:54:14] VERBOSE[2743] logger.c:     -- SIP/50-09ad5dd0 is
ringing
[May 15 16:55:14] VERBOSE[2743] logger.c:     -- SIP/50-09ad5dd0 is
circuit-busy
[May 15 16:55:14] VERBOSE[2743] logger.c:   == Everyone is
busy/congested at this time (1:0/1/0)
[May 15 16:55:14] VERBOSE[2743] logger.c:     -- Executing [50 at intern:2]
Goto("IAX2/jonaskellens-10870", "s-CONGESTION|1") in new
stack
[May 15 16:55:14] VERBOSE[2743] logger.c:     -- Goto
(intern,s-CONGESTION,1)

What is this circuit-busy ?? What triggers this ?? How can I handle
this ??
If the phone was not busy at all... what is then this "circuit" busy ?

Greetingz,
Jonas.
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