here you can find how to solve the problem if you encounter call transfer issues in a scenario with a Spectralink IP-DECT Server 200-400-6500 integrated as a Trusted App in Skype for Business On-Prem.
We have seen this behaviour after upgrading firmware to 18Ba version.
Many thanks to my collegue Paolo Centenaro for doing these tests first.
First of all make sure to have a Full System Backup
Open a second browser tab to this URL https://<SpectralinkServerIP>/config/list
you will find a page like this one
Hit F3 to search in the page for sip.send
to solve the issue you have to change sip.send_bye_with_referfrom true to false,
to do that you have to change the URL from /config/list to /config/set
in Variable insert sip.send_bye_with_refer and in Value inser false, than click Set.
Check the /config/list page, now you will see this configuration
Make some test calls, now the transfer should works correctly.
Hope this could help some of you.