nagios sms

I am trying to find a way to send sms messages whenever an alert is generated in a nagios monitoring system.
The links below contain material that can lead to a solution.
Implementation links:
http://www.lecentre.net/blog/archives/985
http://www.youtube.com/watch?v=oZIB8OAVBtY
http://www.sms-integration.com/p_110-nagios-sms.html
http://matt.bottrell.com.au/archives/205-Nagios-2-way-alerting-via-SMS-Part-2.html
SMS Gateways links for Greece:
http://www.clickatell.com/products/sms_gateway.php?cid=31683
http://www.smsn.gr/index.php?mm=home
http://www.smsgateway.gr/
The first one seems promising as it can provide an SMPT API and can by payed by paypal.
Any of those that provide HTTP API can be used. Clickatel also provides 2 way sms.