Finally ISDN without problems in the IP telephony

After an odyssey with Telecom products (eg W723V, C610 repeater, Eumex800, etc.) I now have a solution that is 100% that does what you want.
From the very beginning: The aim was to continue using the existing telephones with their usual (and cherished functions), which after conversion to VoIP was barely possible (Gigaset 685SX with two other handsets). While I could see the handsets as DECT - register directly on the router, but since this is compulsory in the basement is the absence reach the call on the second floor end. (In addition, the answering machine of Telekom quite annoying !!!).
So shortly before informed (in the network, not at the Telekom!) And then slammed shut. Then unpack Fritz box, disconnect the old router, connect it to phones 'Reset' and the box. Start computer, start the software and wait for the connection analysis (took me about 8 minutes probably ran updates in the background). Internet was immediately available, the installation software is also without a study and even without instructions to understand and operate. Set telephony was surprisingly easy, ISDN - cable in the S0 - login socket, ISDN and delete phones alternatively automatically detected, enter phone numbers, wait for connection testing (time is not even enough for a cup of coffee) and the former DECT - registered phones back to the Gigaset - based sign - ready. Total time about 1 hour (and I am an absolute beginner in this profession). By comparison, only an attempt to log on to the W723V the C610 as a repeater cost in total about 10 hours and then the sch ... system, all nose got out long.
Conclusion: peace at last, everything works again as before switching to VOIP, only faster. Great part, money well spent.