Why should we not use mobile phones in hospitals?

Sources: Google

Don't we always read signs in hospitals saying "Turn of your  Mobile Phone". We do wonder why so!!

Here's the answer for the same:

We know that hospitals use various electronic devices for surgery or other related purposes. When we make or answer a call, electronic waves are sent through the air; these waves try to pass through the electronic devices, and sometimes tend to stop them from working; which can be dangerous. We don't want to disturb a surgery or medical procedure, do we??