I have a similar problem with my Zyxel omni.net. I assume you can connect the modem (or TA) to the modem port, like I do. If you are an expert in Hayes commands, you could write the proper script and then convert it with a free utility "modem script generator"
I got this answer to my query on the Zyxel ISDN TA:
Do you use it is OS9? If you do then you already have a script. I use an Acer T50 ISDN TA and what I did was take the script from OS 9.1 change the creater codes to ???? (use Dropinfo, resedit or whatever) dropped it in the library of OSX for modem scripts and choose it in network settings, works like a charm. If you need any help let me know.