[Serusers] clients not responding to challenges?

[maka]

Hi all,

does anyone know what the reason might be for SIP clients (XLite and
other) on certain Ip addresses not responding to the
www_authenticate() challenges sent by the Sip proxy?

I can see (with ngrep) that the clients send register requests to SER,
and I also see SER sending back challenge packets. Anyway, no
challenge response is sent back from the client, but a normal
REGISTER. XLite diagnostic log does not show any challenge request
packets being received by the client.

My first assumption would be a routing problem between the ISPs
networks, but this all happened suddenly after a restart of my SER
machine, even though nothing on the configuration has been changed.

On some machines(resp. ISPs), the client registers successfully and
can dial out other SIP clients,  but does not receive any incoming
calls. On other - everything works perfectly both ways.

Pls give me any clues or suggestions if u can...