Our application was slightly different, and it too may evolve to text messaging, etc...
when I worked at an electric utility (and I take some credit for initiating this -- I introduced our IT guy to the idea), we had a computer dial up specific pagers and advise our plant management (and officers) when units came on-line, off-line, and some had them paged if a significant load change occurred -- this then developed into a notification of the environmental guys whenever a CEM (continuous emission monitoring) went into alarm...
to the best of my knowledge, they are still using these pagers (and many don't want to give out their cell phone numbers, or be paged via cell phone)
I am not up on the technology developments, but if your business has a dependency on the use of pagers, maybe there is a potential for serving your HVAC/R customers by supplying a paging service of critical customers .... (and I do not know the market either) --