The telephone band of 300-3400 Hz remains the standard compromise for speech intelligibility.

Evidence

The first frequency region suggested for telephones ranged from 250 Hz to 2750 Hz (Martin, 1930), but this was later altered to the range between 300 Hz and 3400 Hz, known as the telephone band. The telephone band was found to be a good compromise between technical constraints and the intelligibility of speech, and it is still in use today. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 2215

Categorization

• CANDY: The bit of trivia that the original phone band was 250-2750 Hz and got shifted to 300-3400 Hz is a nice elevator fact about why phone audio sounds the way it does.
• SHARE: Send to anyone in audio engineering or VoIP — the origin of the 8 kHz sampling rate and telephone band is directly relevant to how modern telephony is designed.