I ran across a free phone-based invitation tool called "Phonevite". The premise of the service is to give you the ability to send a recorded message and blast it out to 25 people with ease (up to 2000 contacts with premium paid account). This is ideal for a personalized invitation or voice announcements. Recently, Phonevite extended its service to be more mobile; "Phonevite to Go" will allow you to setup these voicemail blasts without the need of a computer; you can access the contact group with a unique call in line and drop a mass voicemail over a landline or mobile phone.
I haven't had the need to use this service because: 1). I don't have 25 friends and 2). I never invite anyone over because I am just a lonely blogger. However, if you tried it out for yourself, let me know how it goes :)