Sending a text message or notification to a cellphone

When my business partner or I am not at work, calls are forwarded to one of our personal cell numbers after 3 rings. We periodically alternate who fields calls. I have not determined a way to tell if an incoming call is business-related or personal, which is what I'm hoping to accomplish. My thoughts were either to have an audio or light sensor affixed to the phone that triggers the text/notification.

My question isn't necessarily arduino related, as I have a good idea of how I'd get the sensor working. I just don't know the best way to get the notification to the cell phone in time for the user to notice before their phone starts ringing. Email is probably too slow, and I'm not sure I'd like to pay ~10 bucks a month for an SMS card (I haven't looked into SMS too much, so there are probably cheaper options I haven't stumbled upon). Perhaps a super simple app that I could put on our iPhones?

Thanks for any help! And no, a pager or another cell phone isn't an option. Well, it is, but it's more inconvenient for us than not knowing the source of an incoming call, especially when we are occasionally swapping responsibility for who is fielding calls.

If you want to send SMS messages, you need to pay for a SIM card and monthly service.

I should think it's easier to look at your phone's current CallerID system. If it is a person in your contacts list you probably know what they're calling about and if is isn't, then answer the phone in a professional way.

MorganS:
If it is a person in your contacts list you probably know what they're calling about and if is isn't, then answer the phone in a professional way.

Yes, that's obviously what we've done in the meantime. The reason I'm looking into making this work now, after dealing with it for the last 2 years or so, is that I'd like to be able to forward calls to employees. I don't doubt their professionalism. It'd just be a matter of convenience for them (and me).