Get a remote camera on a snake. I have one with about a 3 foot snake with some different hooks that snap on to snag things. The camera head is about 1/2 inch and the thing costs about $100.
As others have said, the fiber optic camera (Boroscope, Digital Inspection Camera, etc.) is the way to go on this one. They can be rented, but it may take several phone calls and some drive-abouts to find a rental source. Harbor Freight has one you can purchase for $99. Probably the way to go. Might come in handy in the future when an Snickers Bar goes down that duct! Or you want to see what is causing your sink to clog. Team up the fiber optic camera with a gadget to grab hold of a remote object, and Pradeep will be in like flynn !
Fiber optic camera link:
http://www.harborfreight.com/automotive-motorcycle/diagnostics.html
Gadgets used to grab hold of a remote object link:
http://www.harborfreight.com/automotive-motorcycle/specialty-tools.html
ok I have the solution to cellphone that was eaten by the duct. Being an equine veterinarian we get to carry around a lot of fun tools. The perfect tool would be an endoscope (we use them to look way down into the stomach and into the lungs of horses). So I think he should look up the nearest equine veterinarian in his area take the car over and ask the veterinarian if she/he would be so kind as to run the scope down the duct and retreive it with the alligator forceps. Most scopes are 110cm in lenght so it should be a walk in park and a fun challege for the veterinarian.
HAH !
Online get a diagram of actual duct - you will now know how far the phone is lodged in.
Turn on heat MAX for 1/2 hr, this will help to expand the duct ever so slighty. Then, while heater is still running (bend the stick or coat hanger to the specs needed NOT to touch the sides very much). Afix a small suction cup to hanger(cut if neccessary to size - to get it into duct without touching sides), and apply a good amount of superglue to the small suction cup on the end of the hanger. Force it against the phone just a bit - NOT TOO much, wait 2 minutes til the glue is hardened, then pull it out. Or like others, after heating duct, get very small vacuum attachement.
Super glue should work best without much of a mess - then it should actually crack off the phone one retreived.
Good luck Pradeep.
Lets keep it simple.
Buy a small plumbers snake with a grasping claw $10.00 at most hardware stores. Find a friendly tow truck operator put the car on a roll back. and lift it as high as it will go. Use the snake retievw thw phone.
I agree with using a vacuum cleaner to remove the cell phone, but would suggest putting a sock or a pair of pantyhose somewhere in the hose of the vacuum cleaner so the cell phone does not hit the fan of most vacuum cleaners or if using a shopvac the bottom of the shopvac.
Hi Guys , have had similar situations on construction jobs items falling in the heat vents and i’ve retreaved them with the SHOP VAC !! good luck
you can also try a Magnet !!
The suggestion of using a vacuum cleaner is good. I am adding this bit, use the center cardboard tube from a roll of paper towels. Tape one end to the hose, flatten the rest of the tube enough to get into the opening of the vent.
He needs to spend $100 and get one of these:
He can then see EXACTLY where the phone is. He can then take a coat hanger that has been straightened out, with the last inch or so folded over, so that it is thin enough to get past the phone and the folded over part can then hook onto the bottom of the phone and pull it forward.
If you’re still listening Pradeep, and you still haven’t gotten the phone out (or destroyed the car): you need to find a gastroenterologist in your area who will take pity on you. An endoscope of the sort that doctors use to look in a person’s stomach will allow you to see the phone directly as well as retrieve it. In medicine when people swallow things that don’t belong in their stomach this is the easier of two ways to go get it.
If you can’t get a hold of somebody in your locale, you can road trip it up to central New York and I’ll hook you up. 
Use a boroscope and retrieval tool. Then you can guide the tool.