Yes, it's an ATSCALL decoder...
At SWA, MX
doesn't use it, as the dispatcher is the only one capable of initiating contact with the aircraft. The station folks don't use it either. Also, SWA doesn't use it for groups/fleets of aircraft, just each individual tail number.
As an aside, the -300s/-500s/-700s are scheduled to have the ATSCALL boxes yanked out once ACARS installations are complete on these aircraft. When that happens, the dispatcher will simply send a message to come up on a specific location/frequency, and they'll then converse via voice. The -200s will keep their ATSCALL stuff until they're retired, with the last ones going in 2005.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.