Normally the phone frequency is limited, ranging from 300 Hz to 3,4 kHz. Using the AMR-WB codec, speech is transmitted from 80 Hz to 7 kHz.
See: https://en.wikipedia.org/wiki/Wideband_audio
If Ubuntu uses the android GSM/UMTS/LTE driver from android (and the driver supports the codec), does wideband audio work in Ubuntu's phone app?