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.
There probably is a START instruction internally, but it won't take action against a number for which there has been a previous STOP. So UNSTOP acts like FORCE START.
If the process is releasing a STOP (removing or soft-deleting a STOP instruction logged in the DB or some such) then whoever worked on it initially may just have not thought beyond that (especially if English wasn't their first language so unstop might not have sounded any stranger to them than restart). Once something like that is written down and others have seen it, it tends to stick.
Of course it could hav ebeen done by someone like me, who is know to give things technically-correct-but-odd names deliberately…
Unless you need a kidney; then we just bury perfectly good ones regularly, and let the donated ones get a bit more stale while we confirm the opt-in eleventy times.
It's because of the US Constitution and voters' very firm, consistent, coherent stance on bodily autonomy. Even if we consider it to be costing a life, your say over your body is considered absolute, and no federal, state, or local government is allowed to pass legislation that influences what happens to your body.
Just kidding! It's all determined on a case-by-case basis depending on the most conservative perspective of the dominant, favored religion.
