Asterisk & voiceBlue problem

Hello, people.
I have strange problem using asterisk and voiceBlue GSM gateway

in extension.conf i have:

exten => _0903X.,1,Dial(SIP/${EXTEN:0}@gsm,r)
exten => _0903X.,2, HangUp()

sip.conf:

[gsm]
username=xxx
secret=xxx
type=friend
qualify=yes
;permit=192.168.1.251/255.255.255.255
;host=192.168.1.251
host=dynamic
dtmfmode=rfc2833
;disallow=all
context=from-internal
;allow=alaw
;insecure=very
call-limit=1

When calling 0903123456 (gsm operator here) from SPA922 phone, it rings once and then call is cancelled with message in asterisk

– Executing [0903123456@from-internal:1] Dial(“SIP/2512-0821c5a8”, “SIP/0903123456@gsm||r”) in new stack
– Called 0903123456@gsm
– SIP/gsm-0823e588 is making progress passing it to SIP/2512-0821c5a8
– Got SIP response 503 “Service Unavailable” back from 192.168.1.251
– SIP/gsm-0823e588 is circuit-busy
[Oct 30 12:32:56] NOTICE[18690]: cdr.c:434 ast_cdr_free: CDR on channel ‘SIP/gsm-0823e588’ not posted
== Everyone is busy/congested at this time (1:0/1/0)
– Executing [0903602126@from-internal:2] Hangup(“SIP/2512-0821c5a8”, “”) in new stack
== Spawn extension (from-internal, 0903123456, 2) exited non-zero on ‘SIP/2512-0821c5a8’

In GSM gateway, SIP proxy and SIP registrar is is set up to asterisk IP

Have somebody similar problems, just to push me into right direction?

Try to place following fields in sip.conf of [GSM]

fromdomain=gsm
outboundproxy=gsm
host=gsm