I already found this link but am not able to understand correctly what is written there.
I could understand the parts with
"Set Conservative state table optimization"
and
"Disable scrub".
I installed siproxd but the configuration is way to complicated to me.
And I did not understand the part about
"Disable source port rewriting".
What I did was giving the Phone a static dhcp lease.
Then I set up udp forwarding per the directions my provider is handing out.
See:
http://hilfe.telekom.de/hsp/cms/content/HSP/de/3378/FAQ/theme-45859561/Telefonie/theme-45859560/Anschluss-und-Tarife/theme-45859549/Telefonieren-und-Surfen/theme-82239611/IP-basierter-Anschluss/faq-350884716
In short:
UDP (out): Ports 5060, 30000-31000, 40000-41000, 3478, 3479
UDP (in): Ports 5070, 5080, 30000-31000, 40000-41000
TCP (out): Port 80, 443
I only have one phone,
so port forwarding should to the job.
I still have not found how i could forward port ranges,
if someone told my how to do this I would be glad.
Because the wan voip adresses change frequently with my provider I can not use static adressing voip server wise.
Any help would be appreciated.