Ringback (UK)
("Awaiting Answer Indication" or "Ringing Tone")
400 Hz & 450 Hz (50 Hz Beat)
Duration 0.4 s / Silence 0.2 s / Duration 0.4 s / Silence 2.0 s

Telephone Event Tones (US except where indicated)
busy, congestion, dial tone, off hook,
ringback-us, ringback-uk, ringback-japan