The easy/lazy way: Adverts. Advertise on the thankyou page or when the user hits submit. I imagine that your CTR's going to be very low, but you could advertis

The easy/lazy way: Adverts. Advertise on the thankyou page or when the user hits submit. I imagine that your CTR's going to be very low, but you could advertise useful stuff, like phone add-ons (maybe a higher CTR but lower CPC) or new phone contract deals (lower CTR but perhaps higher CPC). The less lazy way to monetize it would be to make the user accept that you will keep their mobile number on file. You can then aggregate that info geographically and theoretically sell it to carriers, if such information was of value to them. The reason this may be of value is that someone who persistently uses your site in a small space of time has probably lost their phone and doesn't know where it is. The telco can then call them up on an alternative number and sell them a replacement phone.