In theory you can send sms with php. However, every script I've found that purports to be able to do this has failed on me. There should be quite a few scripts out there by now. I think you need to know the network provider of the number.
It's easy to send a text message and there are lots of posts that address that. One example.
If you really want to do this as a voice message, it gets (a lot) more complicated. You now need access to a server that has a telephone network connection and an API that lets you send (or receive) messages. Someone has to pay for that so chances are that it is going to cost you something. A couple of possibilities are: Twilio Google Talk
You can do a search and find others.
You might be able to find something off the shelf that does what you want but you're going to have to do some research. If you want to keep it simple, asking for a cell phone number and sending it as a text message is a whole lot quicker and simpler.