AASTRA 4422 IP Office for MD Evolution User Guide - Manual (Page 16)

Introduction to Your Extension
MD Evolution – Dialog 4422 IP Office 16
This ring is distinctly cadenced, according to whether the incoming
call is an internal or external call. The cadence is faster for an internal
call.
Note: Your facility also usually provides integrated on-hold
music, to which every external calling party either placed on
hold or parked will be connected.