Cheewee, has your team looked into sending the messages via emai-to-sms? Most carriers support this and would save you and your users money. You wouldn't have to pay for the outgoing messages and users wouldn't pay for international text.
Verizon users can be reached at [10 digit mobile #]@vtext.com and most other US carriers have something similar.
There are several issues that may arise. I don't know if the codes they send to activate certain features would transfer via email to SMS translation. Second of all what happens with the issue of Google Voice users? Finally, if I steal a cell phone the first thing I would do is change the cell phone number, thus bypassing all security that may be based on a phone number.