To use the features of ISDN also on the router box here, I got an AVM B1 (old, ISA, cheap) for Fax receive/send (hylafax together with c2faxrecv). Additionally, I use vbox3 for voicebox functionality. Works perfectly on this old but reliable IBM P1 200MMX. Debian Packages are available: vbox3, hylafax, as well as capi4hylafax.
But I was unable to check on the monthly phone bill. The rate-de.dat included in the debian isdnlog package includes only CbC (Call by Call) or preselect rates. No data for the Arcor ISDN 760 rates.
Oh, to use this feature, you will need a second ISDN-card in dual-mode. Only certain cards work. I use an old Teles 16.0. You need to re-wire the connection to this card according to these instructions (no liability, it's your own risk!).
Hence, I had to enter it myself. And here you find the rate-de.dat (gzipped). I have not proof-read it and do not assume any liability for my changes, neither the original work by the Tarifdatenbank-Crew. I will probably submit this database later on for inclusion.
To actually use these rates, you need to adjust the rate.conf (usually in /etc/isdn) to show "P:70=3" to use sub-data 3 (which I used in my modifications). Restart isdnlog and you're all set. Hopefully.
Be aware: rates are heavily changing! My rates are based on Arcors Leistungsbeschreibung as of May 28th, 2003. The rates are as of April 1, 2003.
Have fun using it, but let complaints go to /dev/null