Not the OP, but in some countries/with some mobile operators, you can have a contract with “unlimited” (or however rather large) volume of traffic, but for some reasons the traffic is allowed in “chunks”, once you have used (say) 10 GB of traffic, data rate is lowered to a minimum and (it depends on the operator) you need to send a sms to ask for more or a message is sent to you with a code/keyword that allows - sent back by SMS - to “unlock” the next 10 GB at “full speed”.
See also:
http://forum.mikrotik.com/t/script-for-send-sms/156370/1
http://forum.mikrotik.com/t/creating-a-script-for-sending-sms/172627/1