Openvpn Server on a Linux box with bridging (to get in the same network), ssh, ping etc all works great.

Openvpn Client + Applet from extra-devel i guess. (latest)

Asterisk 1.4.17~dfsg-2ubuntu1 on another server in my network.

N900: 1.2009.42-11 (retail)

sip.conf
[XXXXXX]
type=friend
secret=XXXX
callerid="n900" <1234>
canreinvite=no ;needed this!
nat=yes ; and that
host=dynamic
disallow=all
allow=gsm
allow=ulaw
allow=alaw

extensions.conf:
exten => _X.,1,NoOp(${CALLERID(num)} ${EXTEN})
exten => _X.,2,Dial(CAPI/contr1/${EXTEN})
exten => _X.,3,Hangup

i needed canreinvite=no and nat=yes, to get it to work to talk with another sip client (fritzbox).

my problem never was that i cant connect to the other sip telephone, the problem was that i either can hear the other party or talk to the other party

Edit:

as i stated before, i can connect to the asterisk server if i connect to my local wlan. (without openvpn) if i connected to umts or another wlan and the only connection to my network is openvpn i can't connect to the asterisk server.