I don't understand what you are trying to accomplish.
Do you want the use the BT headset as a telephone headset connected via BT to your mobile phone and in the same way, connected via BT to the IP phone? If so, that may or may not work, depending on the headset's Bluetooth version and support for multiple connections. Newer BT headsets should work this way.
This has nothing to do with your DECT headset which is operating via the IP phone's hardwired RJ-9 handset/headset jacks.
Keep in mind that you need to/can control which audio device you use for calling on an IP phone: the phone's own handset, its speaker, a BT headset, a 3.5mm-connected headset or a RJ-9 connected headset.