Telephony protocol sounds

1. dial and busy
2. dial and ring
3. different DTMF tones
4. T.30 fax
5. V.34 modem
6. different answer tones
7. musical instruments

Speech quality degradation

1. original speech
2. after analog telephone (bandwidth reduction)
3. VoIP quality (G.723.1 compression)
4. VoIP over the public Internet (packet loss)