Human discourse is going to pot

At work, I notice this a lot on the phone: I'll end the conversation with a thanks and the person on the other end says in response: "Unh-huh".

Is it just me or is that rude? To me, it can go either way, sounding very snotty or like a neanderthal. Either it's that they're doing you a favor (sometimes when you've done them one), or they can't be bothered to form the words in their mouth.

When a person says thanks, thank you, or thanks a lot, you say "You're welcome". Not "unh-huh" or "mmm-hmm".

Dammit.