Is PABX one and the same to you as IP Office? I wonder why you were switching terms in the first post, and now seem to be differentiating them even more so.
For IP Office 9.1 you can use TAPI. The information related to TAPI is found here
Then click the downloads tab, and then the "TAPI and DevLink"
Look below the table of interfaces for the documentation, SDK, etc.
How well TAPI works on a web server, I don't know. you may need to build a thick client on an application server and interface it to the web server so the thick application can keep up with timers and the like. I am not much of a web server developer.
Release 10.1 of IP Office introduced a Centralized CTI web services interface that could alternatively be leveraged to meet your stated requirements. Its advantage over TAPI is that through one interface you can work with multiple IP Office 500 V2 servers arranged in a small office configuration. With TAPI you have to interface to each 500 V2 independently.
In general DevConnect does not provide free support for IP Office. Should you need further technical assistance I suggest investigating a Support Enabled DevConnect membership so you can get the technical assistance you seek.