I bet this dolphin module actually IS using clickatell.com :)
Just not sure what covers the bill..

After searching around, I find many similar scripts also. (priced from the $ 10 linked above, to $ 400..)
and believe most of them basically grab a free API from clickatell and Sell it..

The reasons sms is free could be one of these:
1. Affiliate system. They get more free sms credited to their account for each site or phonenumber they sign up. (credits can then be used for free sms - or see more sell to users.)
2. Spam system. For a free sms (maybe really 0.056 euro/sms) they get your customers and *could* send them bulk spam and newsletters..
3. Ad system. Txt ad on receiver end. -Not so bad, but ideally the Sender should be the one to view ads :)

Perhaps others also, or a mix..

Here is a short line from one of the sms gateways to clickatell (click and tell? :))
>In order to receive additional SMS credits, members will need to complete offers, registrations or participating in any online activities with that companies that you partner with.
>Receive extra SMS credits for referring friends to signup for a free account

Sounds annoying..

Here's the link to that info, but promise not to buy into it for $400 and give away users eh.. :)
http://www.alstrasoft.com/sms.htm#4
Could risk selling good customers for 0.056 euro a piece it seems.

Say we had 100.000 users phonenumbers from one contry, and signed them all up.
Then went to businesses and told them we could promote their products to 100.000 users directly.. sms is good response usually.
Then sell this about 12-48 times a year.. probably not unheard of.