how to compensate power drains in dc circuits?
They have fixed voltage and current...

They don't! You have usually fixed voltage and a maximum limit for current. When a part of the phone needs more current, total consumption will increase. Since power is product of voltage and current, the power consumption will increase as well. 

When you go over your maximum current limit, your voltage is going to drop. That's how it works in practice. If a system is well designed, then the maximum current limit will be high enough not to be reached in normal operation.