Parking Pro App unable to pickup calls if someone else is also on the main line

User X uses Phone A to take a call on one of the appearances of Line 1

User X uses Phone A to park call 

User Y uses Phone B to take a call on another appearance of Line 1 and is actively on the phone (not parked or on hold)

User X uses Phone C to try and pickup parked call via XML app and the app tries to open and then exits without ever showing the parked calls.  User X cannot use the app to pickup the call until User Y hangs up their call.

If User X dials the parking lot directly (doesn't use the app) - it connects the call to Phone successfully as expected.

All other parking seems to work as well from both phones as long as neither User X or Y is also on the phone with a call.

This did not happen immediately.  When first installed, it was working.  When installed we were using S406's for Phone C - we replaced Phone C with S505 and that seemed to fix the issue, but now 2 weeks later the issue has reappeared with the S505's.