Best Approach for Sending Text Messages from Web App

All,

Anyone have insight/advice/gotachas for the following:

We have a client that provides utility (residential and commercial water) to city residence. We are building an application to allow citizens to see and track their water usage. A new requirement is to ALERT them if their water usage exceeds a user defined limit. The current alert use automated email but now they are requesting the optional feature of sending a text message to the public. Example scenario: your water line breaks while you are away at vacation, you would get a TEXT message stating that your water usage has exceeded 100,000 gallons or whatever your limit is.

I know we can send texts using simple email if you know the carrier (verizon, att, etc) but this requires the user to select their carrier which may change. Anyone have experience texting using any of the WS APIs that return carrier and email information based on a cell number passed in?

Thanks in advance, Paul