In Microsoft Skype for Business Server 2019 or 2015, User A sets Call Transferring to a federated user who is in a different Skype for Business online domain or Skype for Business Server environment. When User B, a PSTN caller, calls user A, and if user A transfers to a federated contact, the call does not transfer. Additionally, a “400 - Malformed Edge Proxy header” error message is logged in the Skype for Business client log.
To fix this issue, install one of the folllowing updates:
The July 2019 cumulative update 7.0.2046.123 for Skype for Business Server 2019, Front End server and Edge server.
The March 2016 cumulative update 6.0.9319.235 for Skype for Business Server 2015, Front End server and Edge server.