To send ether to a contract without a function call:

A: a fallback function must be declared and it must be made payable. If there is no fallback function or the fallback function is not payable it will throw an exception. B: either a fallback function which is payable exists, or no fallback function at all exists. C: you cannot send ether to a contract without explicitly calling a function. The fallback function can never receive ether.