Christopher Elliott, travel commentator, writes, as quoted here in a New York Times article:

(quote)
"My cellphone seldom works anywhere near the hotel," said Mr. T, the senior manager for a ... in ...,Utah. "Not just in my room, but in the parking lot as well."

What is going on? Mr. T., ..., has a long list of probable culprits - including the building's architecture, the area's geography and the cellphone industry's erratic coverage.

But another theory is starting to gain traction among business travelers: hotels are blocking the signals [to encourage more use of the more expensive hotel phones].
(end quote)

It might not be the hotel but rather some independent party. Maybe the speaker at a lecture, annoyed by audience members' voices competing with his, had a jammer and by a push of a button, secured quiet in the hotel ballroom.

Maybe it was the stage manager for a concert or play, hearing unnecessarily loud talking in the balcony, used a jammer.

So we* cell phone users could be to blame, talking when we shouldn't be let alone loudly.

Travel tips:
http://members.aol.com/ajaynejr/travel.htm

*I don't even own a cell phone although on a few occasions have borrowed one.