A student must die.

He's left the room with his phone left in his coat poclet. It's got that frog ringtone. He dies or the phone does. I haven't decided which yet.
In the time it took you to write this post, you could have thrown the phone out of the nearest window.

Who said frogs can't fly?
More specifically: Impel the phone down his throat so hard it comes out of his arse. Sideways.
Smash the phone and then put it carefully back in the coat pocket (ideally leaving no trace of the phone-smashing outside of the pocket.

Attempt to keep a straight face when he returns, puts his coat on, and leaves.

The obvious: answer the phone.

Failing that, move it to the corridor outside, and let natural selection take its course as it gets moved further and further from its starting point. Or possibly goes on a random walk.
Kill them all!

Oh and is it just me or if that frog's cock just ever so slightly disturbing!
Open up the phone and place a small piece of paper across the battery contacts, then replace.
Don't take it out on the phone, it's not the phones fault.
When previously working in an open plan office I seriously considered a mobile phone sin bin. A small, correctly sized, faraday cage which, when the phone is dropped inside, cuts it off from it's base station and renders the office tranquilly quiet once more.

As an alternative turn the phone off and leave this on their monitor.
That had actually occurred to me as well.

Thanks for the notice. May come in very handy.
The last time I saw a Jamster ad in the TV paper, they were proudly describing the frog as the most annoying tone! and seemed to be using its annoyingness as a selling point. I despair of human nature.
I'm astonished anyone has it - I've only ever heard it on the television.
1. Switch it off and/or remove the battery.
2. donate the phone to one of those chairties that ask for old handsets
3. Beat the user to death with any handy implement for the sake of humanity.
Pour coca-cola in his pocket, then you can say somebody else did it because you're boycotting coke. Obviously this has the fatal flaw that you'd have to steal the coke in order to remain boycotting it, unless you use a different brand of phosphoric acid + caramel.
Alternatively you could wait until he comes back and sing Carrots Handbags Cheese in a loud voice over and over until he goes away.
Or you could always change the ringtone to a simple "ring ring" and delete the frog one from the phone's memory.
That's a very practical answer. I'd probably have to torture him as well, just for revenge and/or kicks.

Give his mobile number to as many of those automated dialler scams in the States as you can get and make sure it's on every cold-call list in every call centre in the country. Cold callers and The States don't usually phone until evening so YOU would be safe from the noise.

The ringtone won't last for very long.
So do they call mobiles? It would be cruel but amusing to do that if so.