I've started playing GTA V recently and currently I'm in 39 level. I have pegasus contact in the phone but when I make a call they say that all the operators are busy right now. So, how can I buy stuff from them?
Pegasus only delivers vehicles you've already bought. This includes most military vehicles as well as monster trucks, planes, boats... basically everything you can't have in your garage.
So, before you get to fly a Hydra, you have to buy it from Warstock Cache & Carry, then call Pegasus and order it from them. They'll deliver it to the nearest drop point. Be careful though, vehicles ordered from Pegasus do not have player protection! That means, if you order that Hydra and someone gets into it before you do, it's gone and you either have to steal it back or destroy it so you can reorder it.
Don't worry by the way, there are no insurance costs if a Pegasus vehicle gets destroyed.
It has been a while since I have done this so excuse me if I am wrong, but I believe this is done the same way as buying garaged vehicles. You use the in-game phone to access the Pegasus website and purchase there; only then can you call them to arrange the pickup of the vehicle.
Also, if I recall correctly, certain vehicles from Elitas or Warstock using the same method were accessed via Pegasus. Potentially there is no one "Pegasus" website but separate sites for vehicles that would not normally fit in a player owned garage.