There was a download of CallTTY 2.35.0.481 on the developer's website when we last checked. We cannot confirm if there is a free download of this software available. The software is categorized as Communication Tools.
According to the results of the Google Safe Browsing check, the developer's site is safe. Despite this, we recommend checking the downloaded files with any free antivirus software. This PC software is developed for Windows XP/XP Professional/7/8/10/11 environment, 32-bit version. The CallTTY installer is commonly called CALLTTY.EXE. This software was originally created by DXsoft group. The most popular version among the software users is 2.3.
From the developer:
A program to communicate with phone TTY/TDD devices (Telecommunications Devices for the Deaf) via a sound card, without TTY modem. A simple interface between sound card and phone line or acoustic connection with phone handset are used.
Together with "Virtual Audio Cable" software CallTTY can work via Skype Internet-phone, without additional hardware at all.
You may want to check out more software, such as Active Worlds, USRobotics Cordless Skype Dual Phone or Cordless DUALphone Suite, which might be related to CallTTY.