I want to develop an end user application for Avaya IP Office 9.1

I'm an experienced Delphi programmer and I've been building database applications for the Microsoft platform for many years, but I have not built an app for a phone system ever.

I need to know how to get started. What forum should I be using? What SDK(s) do I need for my development work on my phone system? Where can I find documentation how to get started? With Delphi I can build a Microsoft app, or a web app, but I want to start with a Microsoft app.

Originally I went to: https://support.avaya.com/forums but the link to the "Application Development Forum" but the link is broken.

Thanks in advance for any help you can provide.

Clicking on Products & Resources then navigating to IP Office led me to this page:
https://www.devconnectprogram.com/site/global/products_resources/ip_office/releases/10_1/index.gsp

Clicking on Support & Forums, then All Forums let me see this IP Office forum:
https://www.devconnectprogram.com/forums/forums/show/37.page

That's all the help we can provide on this particular forum, which is not related to IP Office.