A conversation on a "corded" (as they say around here) telephone is usually much easier to hear and understand than the same conversation on a mobile. The difference is caused by the quality of the condenser microphone in the receiver and its proximity to the speaker's mouth, and to the level of interference on the transmission. Wireless transmissions are subject to all kinds of interference that never has access to a wired connection.

Also, I have yet to hear of TTY service (teletype for the deaf) on a mobile ...