Hi there! Full disclosure, I'm a T-Mobile Accessibility manager.
T-Mobile IP Relay service is for anyone in the USA and US Territories - you do not need to be a T-Mobile subscriber, so download away on your mobile device, even tablets. Or you can just use it from any web browser. If you have more questions, you can email IPRelay@t-mobile.com. Hope this helps!
For those of you that can read your cell phone screen or can take the time to transfer the captions to your desktop screen how about reading the details at InnoCaption.com
It is what I use on my cell phone calls both outgoing and incoming. I do not have vision problems in addition to hearing so I read the captions on my cell phone screen. They have been my first choice for several years. There are enough features that I will not try to list here. the web site with details it at https://www.innocaption.com/