If they ask some variation on "How can I transfer your call?" I'll say marketing.
If they say anything else, I respond, "Hi, I'm hoping you can help me out." Then I pause for half a second to see if the GK responds. Whether they respond or not I always say, "I'm a freelance writer and I write marketing materials for companies in the X industry. Who would be the best person to speak with?" Then I shut up.
I get a lot of my numbers from trade show exhibitor lists. The industry I target is large, so there's two or three really big ones (thousands of exhibitors) and at least a dozen smaller shows. If the gatekeeper ask what the call is regarding, I say "I noticed your company purchased booth space at the upcoming XYZ Widgets Exhibition, so I'm calling to see if you need a writer to create <insert types of projects>."
Sometimes the gatekeeper won't know who has that responsibility or they have to look it up. In that case, I apologize for putting them on the spot and say:
"It's usually someone connected with marketing or communications."
Unless the business prohibits the gatekeeper from giving out names or transferring without one, 9 times out of 10 I'll get an answer. They'll either give me the name and either transfer me or tell me I can eliminate the company as a prospect because they have an agency or do everything in-house. Even if I get voice mail, at least I have a name to start off with for the next call.
Hope this helps a few people who are struggling.
Here's a bonus tip: If you get to talk to the decision maker and they tell you anything that doesn't totally eliminate them as a prospect, send them a hand written thank you card. Thank them for taking the time to speak with you, mention a relevant point or two from your conversation, and drop a couple of business cards in the envelope.
I had a prospect who remembered me after two years of no contact because I sent him the card. I wouldn't recommend falling down on your follow up like I did in that case, but I think it shows how powerful this strategy is. Anything that gets you to stick in their mind in a good way.