The CL660HX is an universal handset for addition to DECT/GAP routers or base stations.
The handset is not designed for easy opening. There are a lot of very tight clips in the case that hold the top shell in place.
I used the iFixit Jimmy tool to open the case next to the display a small gap. After that, I used several plectrums to open the case completely.
The device has an extremely annoying red flashing light that cannot be turned off via software. When soft methods don't work, you unfortunately have to resort to harder measures ;-)
I solved it by attaching a RLDD (red light disabling device, some ignorant people also call it a piece of scotch tape) to the back of the keyboard, as shown here in the pictures:
Visual effects for the red light photo were contributed by Frank. Thanks.