Why there is no mobile signals in basement or lift ?

Basically  mobile signals are nothing but the EM waves.

As we know , we are surrounded by EM waves.

Now when are there in basement , we generally does not receive any signals , the reason for this is basement consist of lot of piller and  pillers made up of iron bars .

And iron is nothing but metal. And metals have large number of free electrons.

Once EM wave tries to pass these metals , there will not of energy loss because of more number of free electrons.( This Phenomenon is also called as SKIN Depth).

Same is true in case of lift.

So EM waves loses all its strength.