That’s a pretty specific paging integration challenge, and I can see why you’re trying to replicate the current behavior with the HT813. The current setup with the Avaya system and the 7-ring delay before going to overhead paging is a clear workflow that users expect, so you need to match that exactly.
The HT813 is a solid ATA gateway, and you should be able to replicate that logic with some careful dial plan configuration. The key parts are the ring delay (7 rings) and the two-way audio path, which you’ll need to handle with the right SIP settings and maybe some custom dial rules.
It’s a lot like the challenge of applying UX design principles to marketing. Just as you need to make the user experience of a website intuitive and consistent to keep people engaged, you need to make the paging system’s behavior predictable and logical for the users. If the page goes through after 7 rings, but the HT813 does it after 3, you’ve broken the mental model and confused everyone. The same principle applies: consistency and predictability are key to a good user experience, whether it’s a web page or an office phone system.