When the "Mobile Mountain Study" was conducted by the OnePoll group for the resell site SellCell, it found that nearly half of the people buying a new smartphone hold on to their older device instead of selling or recycling it. SellCell said these unused iPhones are worth nearly $47 billion. It estimated that the older iPhone models account to about $13.4 billion in total.

MarketWatch Stats

According to MarketWatch, the survey through MacRumors claimed 20 percent of those who have not traded their old mobile devices gave them to a family member, partner or friend. About 12 percent were sent to charity and 9 percent thrown away as trash.

For those who have given their old devices to their family members, 22.5 percent were women and 18 percent were men. Security was not a major concern as 18 percent worried about their personal data in old phones compared to 23 percent in the previous year.

Commonly Traded-in Handsets in the U.S.

The following are some of the most commonly traded-in mobile units in the U.S. and their respective estimated recycled values.

1. Apple iPhone 4S 16GB - $110

2. Apple iPhone 4 16GB - $56

3. Apple iPhone 4 86B - $46

4. Apple iPhone 5 16GB - $211

5. Apple iPhone 4S 32GB - $111

6. Apple iPhone 5S 16GB - $325

7. BlackBerry Torch 9800 - $18.75

8. BlackBerry Bold 9900 - $44

9. Samsung Galaxy S3 16GB - $136

10. Samsung Galaxy S4 16GB - $213

Reuse And Recycling program

Reselling handsets has recently become a lucrative business as more consumers move to smartphones while firms search for ways to drive prices down in the emerging markets. Apple is even getting into the action with its reuse and recycling program, a trade-in initiative seeking to boost iPhone sales at Apple retail stores, comprising only 20 percent of iPhone sales. The smartphone trade-in programs became the hottest and biggest area of smartphone transactions in 2013.

Aside from helping to support the environment, consumers shall get credits for returning their unused iPhones that they can use in buying a new one, an affordable way for them to upgrade the most recent model. Potential buyers get attractive incentives to purchase their new mobile devices from Apple store.

Also, the program allows consumers to recycle their non-working iPhones that is environmentally responsible. Eligible devices for the program are iPhone 3G, 3GS, 4, 4S and 5.

Reasons Behind Smartphone Hoarding

Why are many Americans hoarding their unused smartphones? Seventeen per cent of these smartphone owners said they were just "too lazy" to get rid of the devices. Almost 40 percent want to hang on to the old models for spare while 36 percent do not know what to do with them.