Callmanager + Aastra Dect


We have an Aastra Detewe DECT transmitter for our dect phones.
Users use a CallManager (like iSymphony or FOP) to dial from their desktop.
Everything worked fine untill I loaded a new bootversion to our Aastra Detewe Dect transmitter.

Now when users dial from their CallManager it suddenly cannot reach the dect phone they use.
I get this back from the console;

Got SIP response 486 “Busy Here” back from

What could be the solution for this?
Of course I understand that I can do a rollback of the bootversion. But I need a special function which is only available in the newest version.

Help is very much appreciated.