AASTRA 4420 IP Basic (SIP) for MX-ONE User Guide - Manual (Page 12)

Important User Information
Dialog 4420 IP Basic 12
Ring signals
Three different ring signals inform you about the type of the
incoming call. The signals shown in this section are the most used
signals worldwide. Many markets use signals that are adapted for
the local standards. If you get a signal that is not described or you
cannot identify, ask your system administrator.
Internal ring signal
External ring signal
Recall signal
(Callback reminder)