With regard to real callers or paid callers getting on talk radio, I for one did manage to get on one daytime national snow either twice or three times (and I was far from being a paid caller). That show was the Radio Factor (usually when Bill had someone filling in for him. Once was Mike Smerconish and the other was Dom Girodano co hosting with Lis Wiehl). I also got on the air with Lou Dobbs radio show when it was on the air, and earlier this year I was on the air on the Herman Cain show. Some of the other local/regional show I called into and got on the air were Marty Griffin in Pittsburgh, Matt Allen in Providence, JD Hayworth in Phoenix (when his show followed Hannity), Mike Garfield in Houston, Lynn Wooley in Lubbock, Rick Hamada in Honolulu, Jason Lewis (when his show was still locally based) in Minneapolis (twice), Joyce Kaufman in Ft. Lauderdale, Tom Marr in Baltimore, Rocky D in Charleston, SC, Matt Mittan in Asheville, Bob McLain in Greenville, Jen Brien in Boston, Ken Pitman in New Bedford, MA (when his show was on the air), Bud Hedinger in Orlando (twice), Martha Zoeller (when she was syndicated throughout the state of Georgia), Vicki McKenna in Madison, Brian Sussman in San Francisco, Alan Autry (when his show was on the air) in Fresno, Tom Tancredo (when his show was on the air) in Colorado Springs, and even in far flung shows such as John Gormley in Sasketewan, Christy Clark in Vancouver, BC, and Travis Coffman in far flung Guam. Also on two or three occasions I was on the air on Doug McIntyre's Red Eye Radio.
So with regard to national shows, it probably all depends on the show itself individually and not lumped all in one basket on which callers get on the air.