CallTTY TDD software - description from Softdepia.com:
Take to attention that connecting home-made devices to phone line may be prohibited by law in your country. To avoid law violation you can use speaker and microphone mechanically connected to phone handset.
The choice of a signal source (line in or microphone input) and record and output levels are made by the OS ("Volume control" in the system tray).
CallTTY TDD software - description from 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.
CallTTY TDD software is a "Free to try" program. You can download and try it for an evaluation period.