The gateway negotiates with a TTS server using MRCP (Media Resouce Control Protocol) for both TTS and ASR. You can use any product that obeys this protocol. I have only used Scansoft products, which acquired Nuance a few years ago and then adopted that name. You need to calculate the maximum number of concurrent TTS sessions you will be running, and purchase that number of ports. Typically you would get two TTS servers, for fault tolerance, and you probably need to borrow the resources of a third server to install the licence server part as well, so you have a "quorum". This gives you fault tolerance for the FlexLM licencing system.
Very easy to configure on the gateway. Quite easy to set up the TTS system.
Regards,
Geoff