The second claim is not true. I can call with my SIP softphone worldwide (to the "normal" network) at good rates and with good quality. And yes, it already works behind NAT/Firewall without configuring/open the router. And yes, its OSS (kphone), you just have to connect to a SIP provider, which is free as long i don't call outside the SIP network. Im not aware of a German SIP provider where you have to pay for that, dunno how this is handled in other countries.
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The second claim is not true. I can call with my SIP softphone worldwide (to the "normal" network) at good rates and with good quality. And yes, it already works behind NAT/Firewall without configuring/open the router. And yes, its OSS (kphone), you just have to connect to a SIP provider, which is free as long i don't call outside the SIP network. Im not aware of a German SIP provider where you have to pay for that, dunno how this is handled in other countries.
take a look at http://otrs.org/index/
they also provide an online demo.
looks very nice, very versatile and seems to be what you want/need.