[Asterisk-Users] which TDM to use? DID line from telco with no dial tone and no voltage

[hkirrc.patrick]

as my first project with *, i would like to replace our old 
neax2400(sds) with an * server.
i've got an X100p and a TDM400 on hand already.

for the CO lines, the X100p works ok with fxsks signaling though there 
are still strange
things happening every now and again but more testing is on the way.

my real big problem is the DID lines which our telcos call DDI lines;
(incoming calls only)
i disconnected a running DID line from our PBX and did a bunch of
tests on it and found the following:

* line from telco has NO voltage

* the port from pbx is supplying the power(voltage) but no dial tone
 
*the moment i disconnected the DID line from the PBX port,
an alarm is triggered at the telco CO

* i can attach an ordinary analog phone to the PBX PORT, pick up the 
handset and
  send (dial) 4 dtmf digits (being the last 4 digits of our DID number),
  the PBX  will bridge me to the appropriate extension phone.

* if or when the extension phone picks up, the PBX reverses the polarity 
on the line

what type of signaling should i be using for such a line?

many thanks in advance,
patrick