How about a SMS Notification w/o using a 3rd party service or API etc? I know in the US, you can email to text using a phone number. Say for Tmobile you use: PHONENUMBER@tmomail.net And it will send a text to whatever number. AT&T and Sprint do as well. All the addon's I seen you must subscribe to a 3rd party service to use it.
And how do you plan to identify the customer's carrier?
A dropdown/select box? If they select tmobile, just add @tmomail.net to the end of their phone number they put in? Seem's very simple to me.