If you were to call the airline you would want to call united as that is where you booked the flight, but really all that Is needed is the confirmation numbers which should be listed under your united itinerary will have listed other confirmation numbers,

I just recently went through this same thing with united, and online it listed the other 2 confirmation numbers that were on the other non united airlines that I was flying on, Having those confirmation numbers you are then able to check in online on their websites and/or at their gate of the non united airlines.