When I was in Seoul, I tried a couple of the outcall agencies. They are reliable, and send girls straight to your room. Quality and service is variable. I did find a very nice independent Thai girl who I ended up partying at Itaewon with her and her friends, and it turned into an all-nighter with fun afterwards back at my hotel room.
If you have a local Korean buddy, I have heard that the real action is in the KTV karaoke bars.