You should try gammu, it has great documentation and support for modems. Using cell phones seems spotty on whether they will support sending or not, but I think that is independent of the software.
also I chose to get a bigger 8 port modem from here.
http://www.bailing88.com/en/product_view.asp?hw_id=119
I definitely recommend their service.
works great with the latest kernel's and is usb.
seems to price out at about $50 a modem independent the size of the modem pools or a single.