There are short range alphanumeric pagers that might be helpful.
At my workplace I just carried my cell/mobile phone with me and they're text me if I didn't respond to pages/were looking for me etc.
The other thing they'd do is call me and leave a message directly on my voice mail. My cell/mobile providers has (for Hoh/Deaf) a free "voice-to-text" feature using "SpinVox" (part of Dragon Naturally Speaking) which then sends a text message/SMS to my phone with the text version of the spoken voicemail (about 95% accurate) - it's very helpful
There are always ways to make it "work" - it's often just about being creative. I'd have the employee contact a few of the disability services/organizations or Hoh/d/Deaf specific organizations in the area to see if they have any ideas.
Their Audie might also be able to point them in the right direction in terms of organizations and products that might help.