Polycom makes full-duplex boardroom telephone speaker-phones. They use digtal echo cancellers to make full duplex. They have a speaker-phone that uses two telephone lines so that the frequency response extends up to 7kHz so that speech is crisper and clearer.
They have a report about improving speech intelligibility by extending the bandwidth of a telephone call.
The words "failing" and "sailing" are different only above 4kHz. Also the words "Pole, bole, coal, dole, foal, goal, told, hole, molt, mold, noel, bold, yo, roll, colt, sole, dolt, sold, toll, bolt, vole, gold, shoal, and troll".
The very important consonants sounds of speech are from 4kHz to 20kHz. When the consonants sounds are filtered away then all that is left are vowels that do not make clear speech.