Is there a way for a voice call to be converted to text, one person speaking and one person (deaf) only using text

Aside from the already provided answer, have you investigated the Accessibility features on the device yet? Go to SETTINGS > ACCESSIBILITY > HEARING. There is an option for TTY and RTT built into the device along with numerous other features for those that are blind, visually impaired, sensory impaired and hearing impaired.

Rtt and try feature available from iOS 11.2. this will help you. This feature is very useful. Normally this option is available at setting > system > accessibility > RTT turn on off. For Rtt iphone. You have specific requirements and for that you can buy apps from itunes store.

https://ipodtips-tricks.blogspot.com/2018/11/guide-rtt-tty-call-on-iphone-ios-devices.html

Visual Voice Mail is provided by the carrier. Contact them