Again its not always so simple. The ISDN bearer channel is rightly regarded as a switched layer2 technology. But considering that the ISDN network allows a client to open a connection to any point on a meshed network it could be considered to show layer3 characteristics.

Also LAPD which runs on the D channel is considered to be layer 2 (a modification of LAPB). So what is Q.931 which runs over it? This is a layer 3 protocol in a very real sense. The D-channel also supports X.25 packet data over LAPD so again this could be considered to be both Layer 2 and 3.

But if you look at it from the conventional perspective of IP, then these are all just part of a layer 2 process and are therefore all Data Link layer.