AASTRA 4420 IP Basic (SIP) for MX-ONE User Guide - Manual (Page 66)

Installation
Dialog 4420 IP Basic 66
Wall mounting
The phone can be wall mounted, useful for instance in conference
rooms or public areas.
1 Use a screwdriver to remove the handset hook.
2 Turn the hook upside down and insert.
3 Use a screwdriver to remove the two plastic covers.
4 Drill appropriate wall holes according to measurements
given here.
Wall screws Ø max 5mm.