This might fall on deaf ears, but an ability to extend the capabilities of the SMS Tools would be great!
So far SMS gateway devices are often expensive. Having a Mikrotik device (e.g. LtAP) which can interface via API, HTTP or even SMPP would be wonderful since even some software packages are exceedingly dare for little capability.

In some cases, the likes of WISP use with a USB 3G / 4G modem connected to a towers Mikrotik router would be great in cases where say a radio fails or voltage drops significantly enough that it requires an active alert.

I did not understand what you need exactly...
I already let me send voltage warnings or malfunctions via SMS from devices that are no longer connected to the main network...

So far SMS gateway devices are often expensive. Having a Mikrotik device (e.g. LtAP) which can interface via API, HTTP or even SMPP would be wonderful since even some software packages are exceedingly dare for little capability.

The RouterOS's REST API in V7 (or Mikrotik API in V6/V7) should let you invoke the SMS send already AFAIK – not specific to SMS but callable remotely. Similarly, on RouterOS you can use a script that checks temp and send SMS if out of ranch using System>Schedule, or /tool/sms/... can be use in Tools>Netwatch if something isn't pingable.

But basically being generic/API-driven "SMS Gateway" is little different than monitoring specific RouterOS attributes...so @rextended question is pretty relevant since your needs sound solvable today...