Receiving text messages for paging function

Just received an iPhone 3G for Chrismtas. My last two mobile phones, a cheap sanyo and a moto krzr allowed a caller who reached my voicemail to push an option to send me an alphanumeric text. I had been using this as a pager for work. I didn't hear this option when I called my voicemail. I called AT&T and asked them. The rep honestly didn't act like she knew much about the iphone. I've looked for an app and looked on this forum and can't find an answer.

If anyone knows how I can do this I would appreciate it. Otherwise I'll just have to say on my greeting for the person to say the number they want me call back. I know I'll see the number on visual voicemail but when people call from my workplace it rings in as the central number without the extension.

Thanks for any of you gurus can provide!