On trixbox 2.8.0.3, that you can download from here, mISDN is already installed. Hence you will only need to configure it.

[root@pbx ~]# mISDN scan 
1 mISDN compatible device(s) found: 
>> avmfritz 

Great! The card is there. Now lets configure it ... by typing:

[root@pbx ~]# mISDN config 
Writing /etc/mISDN.conf for 1 mISDN compatible device(s): 
>> avmfritz

Super! Lets now configure the drivers to load on startup by editing the script.

[root@pbx ~]# vi /etc/rc.d/init.d/mISDN

When you edit it, either by vi or nano, paste on the second line the below:
# chkconfig: 2345 35 60 
# description: mISDN Kernel Modules
and save the file. Then update chkconfig by typing:

[root@pbx ~]# chkconfig mISDN on

No we are not quite there yet. Few things to change on the configuration generated.

1. Edit the mISDN.conf file. 
[root@pbx ~]# vi /etc/mISDN.conf
<mISDNconf>
<module poll="128" debug="0" timer="no">hfcmulti</module>
<module debug="0" options="0">mISDN_dsp</module>
<devnode user="root" group="root" mode="644">mISDN</devnode>
<card type="avmfritz">
<port mode="te" link="ptp">1</port> --> link="ptp" not ptmp
</card>
</mISDNconf>
Save the file!

2. Edit misdn.conf file (it is not the same file!).
[root@pbx ~]# vi /etc/asterisk/misdn.conf
Add the below:
hdlc=no
[isdn] 
ports=1 
context=from-pstn
Save the file!

3. Edit modules.conf file.
[root@pbx ~]# vi /etc/asterisk/modules.conf
Add the below:
load => res_features.so 
load => chan_misdn.so 
[global] --> This exists!
res_features = yes 
chan_misdn = yes
Save the file!

4. Edit zaptel file (last one, promise!).
[root@pbx ~]# vi /etc/sysconfig/zaptel
You should change it to:
#MODULES="$MODULES wcfxo" # X100P - Single port FXO interface
#X101P - Single port FXO interface
#MODULES="$MODULES wctdm" # TDM400P - Modular FXS/FXO interface (1-4 ports)
#MODULES="$MODULES wcfxs" # either above or this
#MODULES="$MODULES wcusb" # S100U - Single port FXS USB Interface
#MODULES="$MODULES wcfxsusb" # either above or this
#MODULES="$MODULES torisa" # Old Tormenta1 ISA Card
MODULES="$MODULES ztdummy" # UHCI USB Zaptel Timing Only Interface
Save the file!

Restart asterisk, start mISDN (mISDN start). 

Create a Custom Trunk on your pbx using as a Dial String: mISDN/1/$OUTNUM$
You should be all set!