Which SMS Gateway / Service providers are available?
Out of the box, you have two possible providers, clickatell and twilio.

Both seems to work fine, you should check directly on their website because the cost depends on your location, the location of your recipients and the volume you want to send.

https://wiki.civicrm.org/confluence/display/CRMDOC/Setting+up+a+SMS+Provider+for+CiviSMS

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