So a "stopper" can also mean a plug (i.e., something you shove into the neck of a bottle or a pipe to stop things coming out). "Stop" can also then be a verb which means, "put a stopper into"; and "unstop" can mean "remove the stopper from".

Since (it sounds like) this is talking about blocking and unblocking the flow of messages from that number, using "UNSTOP" (remove the thing blocking it) makes more sense than "START"; particularly as the latter seems to imply that you're asking to immediately begin receiving messages, whereas the former simply means to no longer block the messages.