Do you mean you want to connect direct to the ISDN line, or to some output from your phone?

For details of the protocol on the phone line, you will need to read the ITU-T specs - Q.931 etc.
http://www.itu.int/rec/recom...1-199805-I
Or the corresponding national specs for America or Japan.

However, you will almost certainly not be able to do an ISDN protocol stack on an 8051?

If you want to connect to your phone, you will have to read its manual to determine if it provides such an output.
If you're hoping for someone who's done this before, then you'll need to say what phone it is!