AASTRA DT4x2 for BusinessPhone User Guide EN - Manual (Page 23)

23 BusinessPhone – Cordless DT412/DT422/DT432
Description
Signals
Ringing signals are emitted from the phone.
Note: The tones and ringing signals in this User Guide refer
to the standard system but may vary between countries.
Internal ringing signal - repeated after 4 s
External ringing signal - repeated after 4 s
Automatic Callback signal