Sorry to say, but I think you may be hosed with the old equipment.
I looked into this about a year ago and was told by a vendor I could use an Xorcom Astribank unit to connect the old phones to Asterisk as ZAP channels and that it would work.
Unfortunately, it did not come even close to working. From what I understand those old PBX phones are proprietary in most cases and the equipment just does not exist to translate whatever signals they are sending out and interfacing that with Asterisk.
My specific case involved Tadiran phones on an Emerald ICE system. Those phones cannot be hooked up to Analog FXO (or FXS I forget which is which) and have them get a dial tone, much less anything else.
I know that you are probably trying to preserve you initial investment in the phones and to do so you will need to find a piece of equipment that can interface those phones with Asterisk. I looked for a LONG time and found diddly squat.
Just so you are aware, and you probably are, quite a number of VOIP phones have a built-in switch. So if the phones are next to a computer you are covered. I don’t know what the situation is with the phones that are not near Ethernet at all, but you can connect them up over wireless. I have about 12 phones right now that are in sections serviced entirely by wireless bridges and we have had no issues yet. Of course, I put some failover into the wireless bridges too.
If you can’t switch out the phones themselves, all may not be lost… Can you hook up analog lines to that Siemens 9751 and use them for outbound service?
Now if that is possible you could use something like a Xorcom Astribank (they start at 8 ports for around $500 I think) to connect those analog ports to the Siemens 9751. After that I believe it is possible to have Asterisk dial out for you through a VOIP termination provider. Which is pretty darn attractive since I currently pay .9c per minute for termination.
This is certainly not my area of expertise, and you would have to weigh the costs against just replacing the phones entirely, but it might also be possible for you to apply the same concept to the T1 circuit on its own. Essentially taking over the role of your T1 provider. You mentioned that you are a Cisco/Microsoft engineer so you might have a chance of pulling that off. Hook the T1 from the Siemens into the Asterisk box and have Asterisk place those inbound calls back out through VOIP.
There are plenty of T1 cards from Sangoma and Rhino, etc. that allow you to hook a T1 into Asterisk. I just have no idea how to go about configuring that. On the Asterisk dialplan side though, easy as pie.
- EDIT * - I meant termination, not origination. Sorry - it was late.