[Hai-users] Alternatives to C3 Cellular communicator (70A00-2)?

[Eddy]

What alternatives to HAI's C3 "Cellular Communcation Center" exist?

The C3 is unfortunately 2G only, and coverage/functionality is starting to become sporadic because of the "2G Sunset" (http://www.adssecurity.com/support/ATT-2G-sunset.aspx). Right now, I have two C3's (one in Michgan, one in Florida) that currently only accept one single incoming call after initial power up,  then after that, they refuse to answer (there is no indication of any incoming call on the C3 at all).
 
I'm looking for a similar device that will work on 3G (and preferably 4G) networks and allow the same type of "plug and play" functionality as the C3 for both incoming and outgoing calls where regular POTS service is not present.

Thanks for any suggestions.