How can we not miss a call?

Now nearly all people have their own mobile phone. It is convenient for us to keep in touch with our friends, business partners and so on, especially in some urge condition, they can get in touch with us in a shorter time. However we may also miss some important calls for the lack of energy of battery.

In order to avoid it, some people may take a second battery. But in some occasions, both of them may be used up. At that time how can we not miss a call?

It brings a big challenging or opportunity for the manufacture. If they can create a battery that uses the solar to recharge, they will occupy a big part of market. Now the Apple involved researching it. Hope the research can get a good result to bring us more convenience.