Missing extensions after upgrade to Asterisk 1.6

I just upgraded from Asterisk 1.4 (which worked with the same dialplan) to Asterisk 1.6.0. Since then incoming calls from my voip.provider are “rejected because extension not found”.

sip.conf:
[sipgate]
type=friend
secret=password
insecure=invite
username=556677
defaultuser=556677
fromuser=556677
context=sipgate_in
fromdomain=sipgate.de
host=sipgate.de
outboundproxy=proxy.live.sipgate.de
qualify=yes
dtmfmode=rfc2833

[code]
extensions.conf:

[sipgate_in]
exten => _X.,1,Dial(SIP/125,16)
exten => _X.,n,Voicemail(125)[/code]

== Using SIP RTP CoS mark 5 [Jun 6 10:40:03] NOTICE[2345]: chan_sip.c:16869 handle_request_invite: Call from '' to extension '556677' rejected because extension not found.

you need to replace _X with 556677