> In my menuconfig I put an * for :
 > <*> Modular ISDN driver ---> <*>Digital Audio Processing of transparent data
 >                                           <*>ISDN over IP tunnel
 >                                             <*>Support for HFC PCI cards
 >                                           <*>Support for HFC multiport cards (HFC-4S/8S/E1) 
 
try compile as module then load after boot
 
modprobe mISDN_core
modprobe hfcmulti dslot=1
 
 > Is there something more to do for mISDN? I saw on mISDN.org that mISDNv2 is already in the kernel >= 2.6.27 so I were thinking it should work directly. 
 
no it doesn't! it works for ISDN, but not for openbsc. openbsc requires differen SAPI values and multiple layer 2 processes on  the d-channel. it will work with the latest mISDN from git repository:
 
git clone git://git.misdn.org/git/mISDN.git
cd mISDN
sh checkout-branch.sh socket   <- change to socket branch (mISDNv2)
make force  <- delete all mISDN includes from kernel.
make
make install
 
be sure to disable mISDN support from kernel. the modules from git will be installed under /lib/modules/xxxx/extra. the "extra" modules have lower priority than the kernel modules, so disable mISDN in kernel config and reinstall modules (make modules && make modules_install). unused modules will then be removed. then install the git modules as described above.