Possible wrong IP in Connect: header at registration?

I’m trying to set up asterisk to be able receive calls from sipgate.at

It registers OK, but i’m not able to receive any call.
tcpdump-ing shows, that there isn’t any network activity at incoming call.

X-lite SW client works well. So a sniffed registration traffic form asterisk
and X-lite. Here it is:

X-Lite:

SEND TIME: 2420002
SEND >> 217.116.119.252:5060
REGISTER sip:sipgate.at SIP/2.0
Via: SIP/2.0/UDP
91.127.253.141:14319;rport;branch=z9hG4bK90F34CCE63AB4D5CAC1B80F087BEA458
From: sip:6537444@sipgate.at;tag=834140569
To: sip:6537444@sipgate.at
Contact: sip:6537444@[color=blue][b]91.127.253.141[/b][/color]:14319
Call-ID: 739C402D9CA44C5789A19394C988F7BA@sipgate.at
CSeq: 23557 REGISTER
Expires: 1800
Authorization: Digest username=“xxxxxx”,realm=“sipgate.at”,nonce=“4682d224632717388d2c1cb229084e84f1682916”,response=“7407cffe0c8eb0aaf5871ecc73893f53”,uri="sip:sipgate.at"
Max-Forwards: 70
User-Agent: sipgate X-Lite release 1105c
Content-Length: 0

Asterisk

REGISTER sip:sipgate.at SIP/2.0
Via: SIP/2.0/UDP 192.168.0.77:5060;branch=z9hG4bK420e6f36;rport
From: sip:6537444@sipgate.at;tag=as2638140d
To: sip:6537444@sipgate.at
Call-ID: 6de64d7268eb8a1a2a96f8350a2589d8@127.0.0.1
CSeq: 107 REGISTER
User-Agent: Asterisk PBX
Max-Forwards: 70
Authorization: Digest username=“xxxxx”,.realm=“sipgate.at”, algorithm=MD5, uri=“sip:sipgate.at”, nonce=“4682d1d50fd1a2289671efdd226aed2ca67b387e”, response=“74ce7935fde51b921e8eba11b9c589e8”, opaque=""
Expires: 120
Contact: sip:6537444@[color=red][b]192.168.0.77[/b][/color]
Event: registration
Content-Length: 0

Main difference is, that asterisk registers itself with my local IP.
I tried

localnet=192.168.0.0/255.255.255.0

and

externip=xx.xx.xx.xx

or

externhost=my.hostname.org

with no effect.

Any suggestion?
many thanks