7170 S0 with 7270 connect

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I read already some in the forum, am not however unfortunately yet really fndig georden.

I have a 7270v3 with CO2 DSL,
would like over LAN my old 7170 for 2 VOIP numbers to attach and
these then over the s0 exit on my Y-cable of the 7270 as ISDN feed.

1. Can that be done?
2. Who tried out already times?

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Hello,
it would have to go, but without Y-cables.
EDIT: ISDN Out (7170) -> Tel/DSL (7270): ISDN cable
LAN -> LAN1: Ethernetkabel
In this way one has in former times Fritz! Box attached to UFO had.

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I do not want a ISDN telephone to the 7170!!!
Those of the 7270 LAN fed 7170 is to return 2 Sip numbers over s0 into the 7270 as ISDN.
Thus could I transact more than 3 sip calls at the same time in the 7270 - or?
Should be available then in the 7270 altogether 5 lines - theoretically 2 x ISDN and 3 x VOIP.

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With the FB 7270 has the 2 VOIP numbers of the FB7170 with the rules recognized as fixed net, but I could not call over it since busy signal came immediately… no notion why?

Did someone function to such a structure?