Ubuntu :: Ekiga Does Not Accept Incoming Calls?

After I installed Lucid on my Netbook, I have different problems with ekiga.

First of all, some information about my ekiga account:

In ekiga I created a SIP-Account with a usual landline phone-number, my voip-provider is 1&1 (in Germany). In ekiga the account is named "Standard%limit", because I cannot register my sip-account without the "%limit". If I leave the "%limit" out, I get the message "Could not register (Forbidden)", but here on ubuntuforums (or maybe on launchpad) I read about the "%limit" as a workaround. Maybe it has something to do with my problem, I don't know.

Now, this is my problem:

Ekiga does not accept incoming phone calls after a few minutes after launching ekiga. Well, let me describe it this way: Usually ekiga plays a ringtone and shows a message whenever there is an incoming call. However, ekiga only behaves as described and expected just after launching ekiga. A few minutes later, lets say five or ten minutes, ekiga does not accept incoming calls anymore. I tested it with my mobile phone - when I call myself on the landline with my mobile phone, I expect ekiga to play the ringtone and show a message that there is an incoming call. However, ekiga does not do anything and after a few seconds the call is terminated automatically. But it is not the same as if ekiga is not running because if ekiga is not running I get the message that the subscriber is not available.