What should I do after a Sergeant at my AIT Company took my phone and lost it?

Sue the bastard!!!

So what ever happened? Any resolution updates?

It is first of all a sad state of affairs that young Soldiers have to have their phones confiscated. A terrible sign of the times, as we start to see true phone addiction. If the phone was confiscated by your NCO, he/she took on the responsibility for the phone. If it can't be found and the NCO refuses to replace it, you absolutely need to take it up to the next higher level. BLUF: someone may owe you a replacement.

Did the SGT in question expressly said that he lost your phone? Bring the issue up to your AIT Platoon SGT/Drill Sergeant and possibly your unit 1SG (through the Open Door Policy).

Buy a New one.

Learn to live without a cell phone attached to your body. The Greeks, Romans, English, and pre-1970's America built empires without a single cell phone. What value does your cell phone add to your military service?

Go to the IG. But if you were in formation and it went off then blow it off. You were a jerk

Tell the next higher Sergeant.

Go to your post claims office and file a claim.

Embrace the suck cupcake!