the only problem I have with people eating while on the phone is that they tend to take looooong pauses when chewing and swallowing. And that can get weird.
But the slurping thing? I would've just waited until the right moment and said something along the lines of "Hey, are you eating? Can you call me back when you're done? I'm really hungry so I think I'm gonna get something to eat, too..." or, if constantly eating on the phone was a problem, I'd probably say "Do you mind not eating while on the phone? I can't hear you when you're eating."