This saves money for the polling company, but it's hardly national. That's pollster talk. It means calling back to the same person if no one answered the phone the first time. That's important because the selection of people to be polled is at least as critical as speaking to them. A national poll needs to represent a cross section of the population.
Polling organizations are in business to make a profit.
Most polling is not done out of a sense of civic duty, but because they're paid for by politicians, lobby groups or political parties. Their aim is to sway public opinion rather than simply to take a snapshot of the public mood. Responsible journalistic organizations should say where the poll came from. If we can' t find that out, we shouldn't report it. It means attaching a political poll to a market survey so a national poll can be done on the cheap.