Some congressman/senators are busier at night than during the day ;)
Afaik NSA is/was gathering information worldwide.
True, but as congresspeople are elected by the people of the US they would only be keen on hearing from them (well, that's the idea at least - in reality they don't seem to worry a lot about what a small but informed contingent cares about)
Indeed: they only take calls from their own constituents. I, as an Oregonian, will be ignored if I call a representative, from, say, Iowa.