due to a couple of bugs in the ISDN Blaster PALs, the expansion port is not ready to take any expansions by default
Michael Böhmer from E3B describes the required hardware patch
up to 5 ISDN Blasters can be installed into one Amiga
supported by NetBSD
bugs
the card configures with a chained config request, so the Amiga thinks the card in the slot behind the Blaster is located on the same PIC - this bug is mostly harmless and correctable
the card stores only the upper three bits of the base address, assuming that it will get a place in the address space between 0xe80000 and 0xef0000 - this bug is not correctable
the address decoding does not differentiate between the ISDN chipset and the expansion socket - this bug is only noticeable when an expansion module is present; it is correctable