They can and do detect this kind of thing. In a similar vein there is also a whole industry in "sim boxes". Effectively a box of SIM cards / radio equipment that acts as a server. These can similarly be set up as servers to send SMS through carriers ( https://en.wikipedia.org/wiki/SIM_box ), though seemingly the popular use is to bridge VOIP calls to local destinations and sell "minutes" to others. There is also apparently a whole industry in software to manage them. These days that management software allegedly includes measures to have the SIMs behave like humans to evade detection (the sims text each other, browse around, sleep for hours of the day, and so on).