firmware 5.2.9

When FXO connected directly to Aastra 480iCT remote extension (in "FXO Settings"), all incoming FXO calls ring once and go to VM.
FXO log reports that remote extension is busy (BUG).

Fxo_Line0 <0, 0>: OnFxoReportError(), callID = XXX
Fxo_Line0 <0, 0>: Extension XX is busy.
Fxo_Line0 <0, 0>: Cancelling Incoming Ring.
Fxo_Line0 <0, 0>: SetFxoState(OffHook)


Workarounds:
change to "Make and accept only unsecure calls" under extension "Codec Settings"
OR
forward calls through virtual extension