Implications of 2G network demise for basic (SMS/USSD) device communication
I think there is some confusion of terms here.

GSM (TDMA) is one type of 2G cellular network. The other is CDMA. At their most basic, these networks support voice and basic 160 character SMS service.
GPRS is an extension of the 2G GSM network that allows it to transmit data, and GPRS is what enables other features like MMS, push-to-talk, mobile-to-mobile features, and so on. It is colloquially referred to as &quot;2.5G&quot;.
EDGE is what evolved out of GPRS, and allows packet transmission at higher bandwidths.  EDGE is called '2.75G'.

The thing that is being discontinued is all GSM service, regardless of band.  So, for example, AT&amp;T has already sunset their GSM service on both the 850MHz and 1900MHz bands.  The only wireless service now offered by AT&amp;T is as follows:

3G WCDMA/UMTS/HSDPA/HSPA+ - 850/1900MHz Bands 2 &amp; 5 (minimum requirement for nationwide service)

4G LTE –700/850/1900/1700/2100/2300MHz Bands 2, 4, 5, 12, 17, 29, 30, 40, 66
The 2G GSM 850/1900MHz network was sunset as of December 31, 2016; therefore, devices with these specs no longer operate on AT&amp;T’s network.


Whether or not you transmit data or not is irrelevant.  There will be no network listening on those bands anymore.  Voice, emergency calls, SMS, none of it will work at all.  All cellular devices which lack 3G capability will no longer have a cellular network to connect to.  The most legacy network available will require, at minimum, a 3G radio.  So your 2G Enfora and SIM900 will not retain any cellular functionality unless they have the capability to connect to a 3G network. AT&amp;T obviously, has already shut off their GSM network.  Verizon will be shutting off its CDMA 1X network before 31 December 2019.
T-Mobile will be the last bastion of GSM, and will continue offering GSM service until the end of 2020.  After that, GSM will be dead in the United States at least, and just like no ancient analog cellular phones that used the all-analog 1G (original) cellular network won't work anywhere today, neither will GSM radios work anywhere when the world has moved on to 3G+ networks.
Also, this shouldn't be catching anyone off guard.  AT&amp;T has been warning about the shutoff for over 4 years, and likewise with the other networks.
Make sure legacy 2G devices can operate on T-Mobile's GSM bands, as that will extend their life about 2.5 years.  But that should be 2.5 years during which you're upgrading to suitable replacements, not 2.5 years you should be purposefully continuing to use these devices.
I know it's a bummer, but our use or the RF spectrum is total.  As in, we're using it.  All of it.  Everything from a frew khz all the way up to 299GHz.  The RF spectrum ends there because at 300GHz, we stop calling it radio and start calling it infrared light.  
The only way to continue advancing wireless technology is, unfortunately, going to also require discontinuing old services to free up new spectrum allocation. It's worth it though, trust me.