we have a contracted agent that provides an online booking engine.
the results and prices are pretty much identical to what you'd find on public sites,
except they take a (hidden) commission with every booking. (good on them!)
they have a phone line, where they take an additional explicit fee. many people use these, because they dont care at all the booking choices. i like using the online tool, for more visibility and control.
our policy is that EVERY flight booking should go through them. so we can negotiate corporate discounts based on volume. except ive never seen ever advantageous pricing with them (or previous vendors).
hotels and rental cars are available, and are encouraged through that portal, but not mandatory