I have called both Rav Shmuel Kaminetsky Shlita and Rav Reuven Feinstein Shlita. They were both very accessible (came to the phone right away and took time to listen to my question) and easy to talk to.
You can also send sheilas by mail to Rav Chaim Kanievsky Shlita. As far as I know, he answers all questions as long as you only send one at a time. If you are female, you have to either type it up or have a male write the question for you. Also, I think he doesn’t like long questions.