It depends on which Gateway you will be using.
For the Low volume gateways: Enterprise / Online Gateway there is a minimum top up of 25EUR.
See websites: www.cm.nl / www.cmtelecom.com / www.cmtelecom.de / www.cmtelecom.fr / www.cmtelecom.be / www.cmtelecom.co.za / www.cmtelecom.cn
For the High volume gateways: Global Gateway (priority) there is a minimum top up of 250EUR.
See website: www.smsgateway.to
Your credits are valid for one year.