no incoming calls on snom 870

Hello,

because I have to use SIP behind NAT, I run a local siproxd instance.
This has been working fine for my Grandstream device and for
softphones, but with the 870, it doesn't seem to work for incoming
calls. I can dial externally without problems, and the incoming call
is visible in my siproxd log like

Nov 14 13:36:52 nibbler siproxd[4558]: proxy.c:159 INFO:Incoming Call:
asterisk@remoteserverip -> floeff1@mydialupip
Nov 14 13:36:52 nibbler siproxd[4558]: proxy.c:159 INFO:ACK Call:
asterisk@remoteserverip -> floeff1@mydialupip

but the phone doesn't ring at all. I tried various settings, to no
avail. :-( Anyone has an idea on how this might work?

Thanks
Florian

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