So here is what I have:
I have a 6355 in a HPBX configuration just using the 6355 as POE for 15 Aastra 6757i Phones every once in a while the customer calls us saying a phone or phones can see a parked call or a monitored extension.
When a call comes into the main number it rings a MADN line which in turn rings all 15 phones, 2 of these phones have BLF's setup to monitor all the other phones, every phone has 4 enhanced park codes so all 15 phones can park/monitor/pickup the parked calls.
We have transparency on the 6355 turned off at this point and the adtran with 10.9.1 firmware and the phones with a firmware of 3.2.3077. The phones are also vlan'd separating data and phone.
We restart the phone and all is good for a while.
Thanks for posting. I believe this is a situation that we have been troubleshooting through a regular Adtran Technical Support ticket. The notes indicate that the problems persist, regardless of if the SIP proxy feature is turned on or not in the Adtran unit.
Given how verbose the debug output and/or packet capture is likely to be with BLF and ring groups, this is likely something we should continue to troubleshoot with the Technical Support ticket. However, feel free to add any updates or questions you may have on this post. I would be glad to help out however I can.