If you simply made an email signup that said "give us your phone number and choose which carrier from this dropdown to get coupons and discounts on your cell phone", then you could email SMS messages directly to their cell phones using the same software as you send regular emails.

For example, Sprint' SMS email gateway is xxxxxxxxx@messaging.sprintpcs.com (xxxxxxxxxx being the phone number) and Verizon's is something like xxxxxxxxxx@vzwmsgs.com.

so you'd just have to append the correct @whatever.com to their cell number and you'd be done.

If you want them to actually sign up through their phone, I think you have to work that out through the cell carriers.