There is effectively no security in the SMS transmission system. It is quite possible to both insert messages and access messages in the system. Worse, you don't even need to access the server network, as SMS is routed to any device showing the correct SIM code, without any mechanism to verify wether or not the message actually reaches any such device.
Another good choice for an access token system is the tried and true RSA tokens. These appear to be the same devices that Blizzard Entertainment uses for their 2FA. (At least, they appear to be identical and function indistinguishably.)
All the usual pundits insisted that the corporate tax cuts wouldn't result in money coming back to the US or any such things as large corporations raising wages or handing their employees bonuses. Rightup until they started to do both of those things. This is rather like how the actual taxes paid by corporate executives radically rose when Regan cut personal tax rates and suddenly it became much more attractive for those executives to take lots of cash instead of company provided cars, housing, and use of the corporate jet (which were tax-free benefits) and instead use the larger cash payments to buy the housing, cars, and travel they preferred instead of the somewhat less personalized options companies formerly provided. (Oh, yes, this is why senior executives still lived incredibly swanky lives in the insane personal tax regime of the 1950's, it was all paid by the company as 'benefits' of executive positions. Just giving them money is a whole lit simpler than having to maintain the assets required to provide those benefits. And most of them didn't really get what they wanted since you couldn't get a Park Avenue penthouse if the company you worked for didn't happen to own any or it was reserved for someone in a different position.)