Do you know why using a cell phone in a hospital unit could be a problem, when that same hospital provides wireless network service for visitors and patient rooms in the same areas?

I've been told it's because the hospital can define and control the wireless network equipment and the frequencies that the wireless network uses with enough certainty to ensure that there won't be interference with the patient monitoring equipment, but that the uncertainty associated with the way multiple cellular telephone services operate and the variety of handsets makes it more difficult to ensure that there won't be any interference.

The second part of the answer is that if the hospital-provided wireless network causes trouble, then the hospital can turn it off to eliminate the interference. Wireless devices, Wi-Fi computers and cell phones are essentially radio transceivers. The power and frequency signatures of radio transceivers vary significantly as the devices are turned on, used to communicate and turned off.

In mission-critical settings, whether medical or otherwise, the technical teams responsible for service often create conservative failsafe rules and policies to guarantee required levels of essential services.

While people ignore cell phone restrictions on airplanes and in hospitals without immediate negative consequences (just like some people successfully run red lights), it doesn't mean the regulations prohibiting them are not sensible.