It's absolutely ridiculous, is what it is. The Doctor is flighty and scatterbrained and can't stay still for more than five minutes at a time. He's also the most reliable man River knows.
The thing is, eventually, the Doctor will be (has been) pretty much everywhere. It's a game they used to (will) play, back when she first met him. River would name a place or a time or both, and the Doctor would tell her about when he'd been there. Every time, she'd be sure she could think of somewhere or somewhen he hadn't been, but she never did manage it. Eventually she decided he must be lying through his teeth, and told him so. That was when she traveled with him for the first time. To prove it to her, he had said.
Boy, had he ever.
No matter where she leaves a message for him, he's always bound to get it. And he always shows up, no matter what ridiculous thing she needs help with this time. He helped her break out of jail, once, grumbling all the way. The annoyance had been half faked, anyway. She knew him well enough by then to recognize it, and he didn't know her well enough yet to do a better job hiding his glee. At least their continuity issues had some silver linings.
He whooped and spun her around when they made it out, at any rate, which kind of gave it away. He also kissed her, and it must have been the first time for him, judging by the look on his face. Perhaps that had more to do with the hallucinogenic lipstick than anything else. It must have been the first time for him, because he'd remembered (would remember) to wipe it off all the times before.
River Song's universe runs on three constants: The Doctor will always come when she calls, and he will also always make a fool of himself whenever remotely possible. And he'll always make up for it by being completely brilliant.
Breaking into the radio station is a pain, but it will be so worth it, to see what the Doctor has to say about being summoned this way. River's not even sure the TARDIS has a radio at the moment, but she'll probably get one eventually. Or maybe she had one in the past. It doesn't really matter, she decides. River's a patient woman, and she knows he'll show up eventually.