Idect X1 - how to factory reset without PIN

I've bought a second hand Idect X1. The previous owner didn't reset the handset to factory defaults before selling it and now I'm unable to perform the operation (the PIN isn't the default "0000").

I need to do this to access the options to register the handset to my existing DECT base. Is there a key combination/other way to reset the handset without the PIN? I've had no luck on Google.