Asterisk

Discovered another gem in the rough. A open source PBX Server.
Think about it. Voicemail broadcasting. Email read to you. Low cost Vonage like connections. Free phone system for your office.

Go to http://nerdvittles.com/

They have tuned this setup for even Windows folks like me.

Also get the Asterisk book download published by OReilly. It is a great Nutshell book for writing code against it.

Then if you code in .Net C# like me... You will want to go one of two ways.

First there is a person who wrote a C# dll to interface with Asterisk.
http://www.gotdotnet.com/codegallery/codegallery.aspx?id=7948963b-7a3b-4303-9118-da800f4f84c3
I suspect if you want more control though you will want to try this idea out like I am currently...
http://www.codeproject.com/cs/internet/simple_tcp_client.asp
This should create a simple TCP client that can Listen and Send. I haven't tested it yet but I will let you know more later.
Asterisk does supply the complete ability to do almost all the work that you need through reloading .CONF files... You still need to run the reload command though.

Hope this all helps... PS:Think about advertising mixed with Asterisk and GeoCode data. Wow the possibilities are endless.

Peace!