I think ClockworkSMS is not really stable, and the portal is really annoying. They ask you to fill in your mobile phone number to receive some free credits. A verification code would be send per SMS, but it never arrived (after several attempts). I think it's very poor that a SMS sent from a SMS gateway provider doesn't arrive. Because it didn't worked, I pressed the 'contact us' link: it was broken. To test ClockWork I wanted to add some credit. I started a payment via PayPal but saw that I forgot to add my VAT-number. I canceled the payment, added my VAT-number and started the payment again. When I finished the payment I got an error message (at the very end) "Too much payment attempts made from this IP" . In other words: SMS deliveries are unreliable and their portal is annoying.