Some wall mount hooks are a 'slide out' fitting. Try this.
Remove the handset from the cradle. Gently press the hook insert and slide it up towards the large 'hang up' button. Rotate the hook insert through a half turn and slide back into the slot. This should create a hook for the handset.
Press and hold the zero! I have googled this so many times and always have to download the manual for something that needs a 5 word sentence. I hope this helps someone else.