AASTRA 4225 for MX-ONE Telephony Switch - Manual (Page 124)

Installation
Dialog 4223 Professional/Dialog 4225 Vision 124
Installation
Install cables
You can put the cable to the handset into the notch underneath the
telephone. The cable to the exchange has to be plugged into “LINE”
and the handset cable has to be plugged into “HANDSET”.
Change cables
To remove a cable, use a screwdriver to unlock the stop.
Handset cable to the left Handset cable to the right