Typically no.

The hospital staff always shakes a finger at you and says something about how cell phones interfere with sensitive instruments. They never say which instruments, or how exactly the interference happens.

So do cell phones really affect hospital equipment?

Cell phones do interfere with a whole list of electronic devices: car radios, televisions, land line telephones, airline communications equipment and even wireless Internet signals.

In the US for example, the FCC warns that cell phones interfere with medical gadgets. 

Because of this, hospitals err on the side of safety.

Whether they should is another matter. Four years of testing done by the Mayo Clinic has shown that cell phone signals actually don't really interfere much at all with hospital equipment, primarily because of technological improvements in cell phones and hospital equipment. 

In fact, the two-way radios used by emergency personal interfere a great deal more than cell phones do, but never to the point that a patient has been affected.

This article argues that cell phones should be allowed in hospitals for the sake of conveying critical information to doctors instead of relying on paging systems.

In other words, the prohibition against mobile phones in hospitals may do more harm than good.