After I unboxed my T-Mobile HTC One S and played around with the camera for a bit, I finally got around to heading over to my local T-Mobile store, where I bought a SIM card for the phone. (I got it off-contract from a third-party retailer.) But then, once it was activated, the most annoying thing happened: a voicemail notification popped up that I couldn't get rid of.
I tried clearing all my notifications: it wouldn't go away. I set up my voicemail account, and made sure the inbox was empty, but it was still there. Interestingly enough, this is actually a known problem with other HTC devices on T-Mobile, so it's a shame to see that it hasn't been fixed yet.
Eventually, I just decided to call T-Mobile customer service and remove voicemail entirely from my account; I'm using Google Voice anyway, so there's really no reason to have T-Mobile's voicemail, too. Before I had even hung up, the voicemail notification was gone – so that's one way to get rid of it, if you're experiencing the same issue.
However, if you actually want to use T-Mobile's voicemail service, there's another way to get rid of this annoyance: just call your cell number from another phone, leave yourself a message, retrieve your message, and then delete it. It's as simple as that, and you're good to go until you factory reset your phone.
Yes, this is really annoying. I know. But in the grand scheme of things, it's a headache that can go away in under five minutes, so it hopefully won't detract from an otherwise awesome device.