IVM is free for a trial period only, after that you must purchase a license, and because you want the outbound dialing feature, you will need at least the Small Interactive System license (see here for different licenses http://nch.com.au/action/reginfo.html#ivm).
You will be able to have other programs make IVM call out and deliver a message, for example:
Execute: IVM.exe -outbound 5551234567 -set “testing=haha this is my message"
And then have outbound calls play an Out Going Message that is set to read out what is stored in the variable testing.
So all you need is away to get your webserver to execute the IVM program with the appropriate arguments and it should work fine.