Steve, what type of PBX are you registering to? We haven't tested 5.3 with ADTRAN PBXs, and it's possible some functionality was changed or removed. We do have a workaround that allows you to add an EFK to Polycom phones specifically for blind transfers if that would suffice:
Adding a Blind Transfer Softkey to Polycom Phones
If not, please provide as much detailed information as possible about the specific sequence of button presses the end users are using to complete a blind transfer and what has changed after the upgrade. Thanks
