700-900MHz sounds like a "wideband" splitter, in that, for its design, it works over a relatively wide range... but still, it's a tuned design (perhaps with 1/4 wave elements?), and only works over that range.
If that's what it is, I would expect it behaves as a lump of metal at very low frequencies (i.e., all ports shorted together, so the mismatch at all ports sucks, and the coupling coefficient is equal). Or if it's a fractional power splitter, like the kind with coupled 1/4 wave traces, the coupling coefficient will be 1.0 for the "through" ports and 0.0 for the "coupled" ports.
The (truly) wideband splitters use transformers with permeable cores to enforce matching over a very wide range of frequencies (several decades).
So, the correct answer is: depends on design, but most likely, it's not going to do anything like what it does in the rated bandwidth, and there's probably a good reason why, even if we don't know what's inside the black box.
Tim