Integrating automatic phone call solution

I have a Windows service running in .Net which needs to be able to call a number and play a wav file. The windows service will be running on the same box the modem is on.

I’m not sure what I all need. I need to be developing and testing on a different pc. Also, the voice gateway and modem may need to be running on two PCs to eliminate a single point of failure.

What software and licences are needed and what is the cost?

All you need are two Voicent Gateways deployed on two PCs. Each gateway can have a single phone line or multple phone lines, depending on your phone call volume. To start, just get two licenses for Voicent Gateway Outbound 1 line software. Price info is available at Voicent online store

From your .Net application, send the call request to the first gateway, if failed, then send to the second one.

Voicent Gateway tutorials, samples, and document are available under developer section