Please refer the 3cx manual there you can find few free tools available for test-to-speech. try this link for online text-to-speech
http://www.research.att.com/~ttsweb/tts/demo.php
Good and Free in telephony usually means Linux, Asterisk and Festival. This is also hard and time consuming. Good and free and easy TTS does not exsist in telephony, because there is alot of money to be made.
For windows, free and kinda good usualy means you need to learn to integrate SAPI into your voice app and use Microsoft TTS. This is also not that easy to do.
Good & fast and easy to build usually means you need to pay $$$ and get a TTS toolkit specifically for telephony that integrates support for the major commecrical TTS engines. eg: VBVoice toolit + Speechify TTS engine = drag prompt on screen, type text. Voila text to speech in voip telephony app.