ekiga twinkle VoIP and internet configuration

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http://forums.opensuse.org – Hi, I tried ekiga after that tried twinkle as VoIP application but have one problem with both - at first everything is fine both program registered my account and work fine - can make and receive calls, after some time I can make only calls , they do not accept incoming calls, there is no rings or any signal that there is incoming call. Restarting application the situation is the same - it accepts calls in the begging and after that stops. Most probably I have to make some adjustment on the internet connection, it seems it closes incoming calls after a time , but I do not know what should be done. My internet connection is direct, LAN , no proxy. Any help will be useful. (Distributions)