I have deployed a SFB 2019 and Lync 2013 pools in the same AD forest. Persistent chat is enabled on Lync 2013 server and it is not supported in SFB 2019 pool. This blog will test the client operation between the environments. Here is the topology for reference;

This blog may help you to test and implement static registration expiry for SFB mobile devices. Static registration configuration is populated in web.config file under following location. Usually, Microsoft do not support any configuration changes to webconfig file until or unless recommended by support.

This blog may help you to understand the expected workflow for troubleshooting the on-premise PPT sharing issues with SFB 2015 deployment.

First of all,  tri co-existence is not supported by Microsoft. This article doesn’t endorse any unsupported configuration.

There are many cases where in user left the company and rejoined or migrated from resource forest etc can cause AD SID mismatch in FE server SQL express database. Later, user may not be able to signin to SFB client. Following steps may help you to delete the orphan object and clean the FE database.

This a blog may help you to narrow down the issue. I have see above issue with few customers recently.

Skype for business CQD on-premise deployment by default will not add the category for third party endpoints. Following article may be helpful for adding a new category for third party endpoints.

Skype for business server fabric traces are enabled by default. You can find the fabric traces in following location; C:\ProgramData\Windows Fabric\Log\Traces. There is a possibility, some strange fabric issues can fill up your system drive space. Following article may help you to reduce the fabric traces to one day.

This document may be helpful when setting up SQL profiler tracing on Lync/SFB servers.

This article may help you to understand the client behavior when adding extension in skype for business 2016 contact card. I have installed SFB 2016 version 16.0.4266.1001 and SFB 2015 CU3 as backend.