MSHA has issued a Program Policy Letter that provides:

mine operators guidance in implementing: (1) alternatives to fully wireless post-accident two-way communication between underground and surface personnel and (2) electronic tracking systems, both of which are required by the MINER Act. The two-way communication alternatives (or “partially wireless” systems) include infrastructure underground to provide untethered communications with miners.

The complete PPL may be found here, http://www.msha.gov/regs/complian/ppls/2011/PPL11-V-13.asp