I don't have a problem with "Beat", it communicated just fine with me. It could have said "He paused for a moment," but "Beat" works. I don't think I would use it, but I understood it, and I didn't find it jarring. There is missing punctuation in the dialog though, I think it should be:
"... I’ll let Glenda know what’s going on too, so she can head Jim off at the pass.” Beat. “Um…I guess that’s it. I’ll be here till two, I guess, if you want to call me. See ya.”
Without the commas around "I guess", it looks like a mess.